- Demand — how much buyers want at each price
- Supply — how much sellers offer at each price
- Equilibrium — where supply meets demand
Fish in the sea, buyers on the beach
On 14 January 1997, at a beach called Badagara on the coast of Kerala, in southern India, 11 fishermen threw their catch back into the sea. The beach market had more sardines than its buyers wanted, and fish cannot wait. On the same morning, there were 27 buyers at other beach markets within 15 km who would have bought those fish. They went home without them. (Source: R. Jensen, "The Digital Provide", Quarterly Journal of Economics, 2007, as reported in The Economist, May 2007.)
Look at what was there that morning. There were sellers with fish. There were buyers who wanted fish. And still no fish changed hands, and good food was thrown away.
So something was missing. It was not fish, and it was not buyers. It was the thing that connects them. Economists call that thing a market. This lecture is about what a market really is, why buyers and sellers bother to trade at all, and how a price gets found when nobody is in charge. At the end, you will see what changed on that coast after 1997.
The last lecture showed that prices in a market economy signal, reward and share out scarce goods. It promised that the next lectures would look closely at how markets work, through demand and supply. This is the first of them, because demand and supply describe a market.
A market is not a place
When people hear the word "market", most of them picture a place: stalls, crowds, fruit in baskets. That is one kind of market. But think about someone selling a used bicycle online. There is no building and no stall, and the buyer may be in another town. It still works the same way.
So in economics, the word means something wider.
A market is any arrangement that brings buyers and sellers together, so that they can exchange a good, a service or a resource, and agree on a price.
Every market has four parts. There are buyers. There are sellers. There is something being exchanged. And there is a way for them to find each other and agree the terms: a stall, a website, a phone call, an auction, an advertisement. The fourth part is the one people forget. Without it, as on the Kerala beaches, buyers and sellers can be close together and still not trade.
Economists usually talk about the market for one thing: the market for rice, or for used bicycles, or for nurses. Some markets are local, such as haircuts in one town. Others are worldwide, such as gold, which is easy to carry and the same everywhere.
Some markets do not look like markets at all. In the labour market, workers sell their time and skill, employers buy them, and the price is the wage. At a money changer, one currency buys another, and the price is the exchange rate. When one family swaps mangoes for another family's eggs, there is no money, but there is still an agreed price: so many mangoes for so many eggs. Trading goods for goods is called barter.
Now test the definition. Some of the cases below are markets, and some only look like them.
Put each card in a box. Ask two questions: are people choosing to exchange something, and do both sides agree the terms?
This box sorts eight cases into markets and non-markets, with the reason for each.
A market: a fish auction on a beach every morning. A website where students sell used textbooks. A factory hiring 50 workers at an agreed wage (the labour market). A traveller changing rupees into dollars (the currency market). Two neighbours swapping mangoes for eggs (barter).
Not a market: a family sharing dinner at home (nothing is exchanged for a price). A school giving every student free textbooks (a rule hands them out). A government office ordering a factory to make 5 000 tractors (a plan decides).
You should have found that neither a building nor money decides the answer. What every market has is a free choice on both sides, and terms that both sides accept.
Why trade at all? Because both sides gain
Here is a question that sounds too simple. Why does anyone trade?
Many people think trade is a contest: whatever the seller wins, the buyer loses. That idea is wrong, and seeing why is the key to this topic.
Every buyer has a highest price they would pay. Above it, they would rather keep their money. Every seller has a lowest price they would accept. Below it, they would rather keep the good. A trade can only happen when the buyer's highest price is at least the seller's lowest price.
In an invented town, Ana wants a bicycle to get to school. The most she would pay for this one is $90. Ben is selling it. He has a new one, and the lowest price he would accept is $60. They agree on $75.
Think first. Is Ana better off, or Ben, or both?
Ana valued the bicycle at $90 and paid $75. She is better off by .
Ben valued keeping it at $60 and received $75. He is better off by .
Both are better off. Together, they are better off, which is the gap between Ana's highest price and Ben's lowest price: .
Check. Any price from $60 to $90 leaves neither of them worse off. At $95, Ana would walk away. At $55, Ben would. The trade happens only inside the gap. ✓
That is it. That is the whole idea. When people trade freely, both sides expect to gain, or the trade would not happen. The price does not decide whether there is a gain. It decides how the gain is shared. At $65, Ana gains more and Ben less. At $85, it is the other way round. But the $30 gain exists only because the bicycle moved from someone who valued it at $60 to someone who valued it at $90.
This is why markets matter. A market moves goods to the people who value them most, and every move creates a gain. On the Kerala coast, sardines that buyers 15 km away wanted were worth nothing in the sea.
Ana and Ben both gained $15. A friend says: "That is impossible. Ana paid Ben $75. Money cannot appear from nowhere, so one of them must have lost." What is wrong with the friend's argument?
Show the answer
The friend is only counting money. The $75 moved from Ana to Ben, and the bicycle moved from Ben to Ana. What changed is who has the bicycle. Ben valued it at $60 and Ana at $90, so it is now worth more to its owner than before. The gain comes from that move, not from the money. This is why a voluntary trade is not a contest with a winner and a loser.
How a price gets found
Nobody tells Ben what to charge. So how does he find a price? He searches, and the buyers search with him.
In an invented town, Priya is selling a used phone online. She does not know what it is worth. Four people are looking for a phone like hers. The highest prices they would pay are $170, $165, $150 and $120. Priya would not sell below $100. She does not know any of these numbers.
| Week | Priya's asking price | Buyers willing to pay it | What happens |
|---|---|---|---|
| 1 | $200 | 0 | No messages. Priya lowers the price. |
| 2 | $180 | 0 | Still nothing. She lowers it again. |
| 3 | $160 | 2 | Two buyers want it. She sells it. |
Think first. Why did Priya keep lowering her price, and could she have done better in week 3?
Weeks 1 and 2 sent her a message: at that price, no buyer gains, so nobody replies. Silence is information. In week 3, two buyers wanted the phone at $160: there were more buyers than phones. If Priya had asked for offers, the two buyers would have competed. The buyer with a highest price of $170 could beat the other one by offering a little more than $165. So the phone could have sold for about $166.
Check. At $166, the buyer still gains (), and Priya gains more than at $160. ✓ No price above $170 works, because then no buyer gains. ✓
Look at the two forces in this example. When nobody wants to buy, the seller's price falls. When more than one buyer wants the same thing, the buyers push the price up. Nobody set the final price. It was found by the two sides searching from opposite directions. Economists call this price discovery.
In a big market, the same search happens all the time. Later lectures turn it into two lists: how much buyers want at each price (demand), and how much sellers offer at each price (supply). For now, hold on to the picture: a price is not set by one person. It is discovered by both sides.
Competition limits what any one seller can charge
"But surely the seller sets the price," you might say. "The noodle stall writes $4 on the board, and I pay $4." In one sense, yes. But suppose it wrote $12, while every other stall nearby charged $3.50 to $4. Almost nobody would buy there. The stall can choose any number it likes, but the buyers choose whether to pay it.
So a seller's power over the price depends on how many other choices the buyers have. With many stalls selling similar noodles, each one has very little power. The same works the other way: a buyer has little power when many other buyers want the same thing, as Priya's phone showed.
Now compare the only shop on a small island, with no other shop for 50 km. It can charge much more before buyers stop coming, because their next choice is a long boat trip. The reason is not greed. It is the lack of competition. Competition, many sellers trying to win the same buyers, is what keeps prices close to what buyers will accept. Later lectures look at what happens when there is very little of it.
A real case: mobile phones on the Kerala coast
The situation. The economist Robert Jensen studied 15 beach markets along about 225 km of the northern Kerala coast. Each boat sold its catch at its home beach. The markets were open only for a few hours, so a boat could not visit several beaches to compare prices. When one beach had a large catch and few buyers, the price fell very low, and some fish could not be sold at all. Before 1997, about 5% to 8% of the catch was thrown away.
The change. From 1997, mobile phone towers were built along the coast, and the signal reached boats about 20 to 25 km out at sea. Coverage arrived in different parts of the coast in different years, up to about 2000. Fishermen bought phones. While still at sea, they could now call several beach markets and ask about prices before choosing where to land.
The prediction. This lecture predicts that connecting buyers and sellers turns several small, separate markets into one bigger market. Boats should go where buyers are waiting. Prices at different beaches should move closer together. Fewer fish should be wasted, because a catch that is not wanted on one beach can be sold on another.
What happened.
| Northern Kerala, sardines | Before phones | After phones |
|---|---|---|
| Share of the catch thrown away | about 5–8% | almost none |
| Fishermen selling outside their home market | none | about 35% |
| Daily price differences between beaches | large | about a quarter of before |
| Fishermen's profits | rose by about 8% on average | |
| Prices paid by consumers | fell by about 4% on average | |
(Sources: R. Jensen, "The Digital Provide: Information (Technology), Market Performance, and Welfare in the South Indian Fisheries Sector", Quarterly Journal of Economics, vol. 122, no. 3, August 2007; summarised in CORE, The Economy, Unit 8.11.)
Verdict. The prediction fits very well. The phones reached different parts of the coast in different years, and the changes came as each region was connected. That is strong evidence that the phones caused them. Fishermen and consumers both gained, at the same time, from the same fish. That is the gain from trade in Worked Example 1, repeated thousands of times. Two cautions are fair. Not everyone gained: the fish traders, who had bought cheaply from boats that had no choice, lost some of their power over the price. And the study is about one fish, on one coast, in the years when phones first arrived. Still, it is one of the clearest pieces of evidence that a market is not only buyers and sellers. It is also the information that lets them find each other.
The model below lets you see the same idea with numbers. It is invented and deliberately simple.
An invented coast has three beach markets. Today, 100 boxes of fish come in at beach B and 60 at beach C. Choose how many land at beach A. At each beach, the more fish there are, the lower the price buyers will pay, and buyers will take at most 160 boxes. Without phones, each boat sells at its home beach. With phones, boats go where the price is best.
Try:
This table shows an invented coast with three beach markets. Beach B's boats catch 100 boxes and beach C's catch 60. At each beach, the price per box is $30 minus 15 cents for every box landed there, up to 160 boxes. Buyers take at most 160 boxes, so the price never falls below $6. Without phones, each catch is sold at home. With phones, boats move until the price is the same at every beach.
| Beach A's catch | Phones? | Price at A, B, C ($ per box) | Boxes thrown away | Paid to all fishermen |
|---|---|---|---|---|
| 40 | No | 24.00, 15.00, 21.00 | 0 | $3 720 |
| 40 | Yes | 20.00 at every beach | 0 | $4 000 |
| 100 | No | 15.00, 15.00, 21.00 | 0 | $4 260 |
| 100 | Yes | 17.00 at every beach | 0 | $4 420 |
| 200 | No | 6.00, 15.00, 21.00 | 40 | $3 720 |
| 200 | Yes | 12.00 at every beach | 0 | $4 320 |
With phones, prices become equal, no fish are thrown away, and the fishermen together earn more from the same catch. The model leaves out the fuel and time needed to reach another beach, the traders in between, and the cost of the phones.
You should have seen three things. Without phones, a big catch at one beach crashes the price there and wastes fish. With phones, prices become equal, nothing is wasted, and the fishermen together earn more from the same fish. And not everyone gains: with a big catch at A, buyers at beach A lose their very cheap fish, and boats from beach C get less than before. A better market makes the total gain bigger, not every single person better off.
Three traps
A market is any arrangement that lets buyers and sellers find each other and agree a price. It can be a beach, a website, a phone call or a job advertisement. In an exam answer, define a market by its buyers, sellers and exchange, not by a building.
A seller can choose the number on the label, but buyers choose whether to pay it. A price is discovered by both sides, and a seller's power depends on how much competition there is.
In a voluntary trade, both sides expect to gain, or one of them would say no. The gain comes from the good moving to someone who values it more. The price only decides how that gain is shared.
Work each one on paper first. Open a hint only when you are stuck.
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For each market, name the buyers, the sellers, what is exchanged and what the price is called. (a) A tuition teacher in Singapore who gives lessons to students for $40 an hour. (b) A tourist in Barbados changing pounds into Barbados dollars. (c) A rice farmer in Chennai selling her crop to a trader.
Hint 1
In each case, ask who pays and who receives the payment.
Hint 2
In (a), the teacher is selling time and skill. In (b), one currency is used to buy another.
Full answer
(a) Buyers: the students (or their families). Seller: the teacher. Exchanged: lessons, the teacher's time and skill. Price: the fee, $40 an hour. This is part of the labour market for teaching. (b) Buyer: the tourist, who buys Barbados dollars. Seller: the money changer. Exchanged: one currency for another. Price: the exchange rate. (c) Buyer: the trader. Seller: the farmer. Exchanged: rice. Price: the price per kilogram (or per tonne). Check: each market has all four parts: buyers, sellers, something exchanged, and a way to agree the terms. ✓
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In an invented town, Kofi wants to buy a second-hand guitar. The most he would pay is $45. The owner, Mira, would not sell for less than $30. (a) Between which prices can they trade? (b) If they agree on $40, how much does each of them gain? (c) If Mira's lowest price were $50 instead, what would happen, and why?
Hint 1
A trade happens only if the buyer's highest price is at least the seller's lowest price.
Hint 2
Kofi gains his highest price minus the price paid. Mira gains the price paid minus her lowest price.
Full answer
(a) Any price from $30 to $45. (b) Kofi gains . Mira gains . Together they gain . ✓ (c) No trade. Kofi will pay at most $45, and Mira will take at least $50, so no price works for both. Mira values the guitar more than Kofi does, so it is best that she keeps it. No trade is the right result here. Check: the total gain is always the gap between the two limits, whatever the price. ✓
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Wei is selling a used camera. Three buyers are looking. The highest prices they would pay are $120, $110 and $95. Wei asks $150 in week 1 and lowers the price by $10 every week until someone buys at the asking price. (a) In which week does she sell, and to whom? (b) In that week, is there more than one buyer? (c) Could she have sold for more?
Hint 1
Write the asking price for each week: $150, $140, $130, …
Hint 2
A buyer buys in the first week when the asking price is at or below their highest price.
Full answer
(a) The asking prices are $150, $140, $130 and then $120 in week 4. In week 4, the first buyer, whose highest price is $120, is willing to pay it. So she sells in week 4 for $120, to that buyer. (b) No. At $120, only the first buyer is willing. The second buyer's highest price is $110. (c) No. Nobody would pay more than $120, so $120 is the most she could get. A bidding contest between the first two buyers would have ended a little above $110, because the first buyer only needs to beat the second. So her falling price did better than an auction here. Check: $120 is at or below the first buyer's highest price, and above every other buyer's. ✓
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A stretch. Use the rule from the three-beach model: the price per box at a beach is $30 minus 15 cents for every box landed there, and buyers take at most 160 boxes. Today, beach A's boats catch 160 boxes, beach B's catch 100 and beach C's catch 40. (a) Without phones, find the price at each beach and the total paid to all fishermen. (b) With phones, boats move until the price is the same everywhere. Find that price and the new total. (c) Who gains and who loses from the phones?
Hint 1
Without phones, the price at A is . Multiply each price by the boxes sold there, then add.
Hint 2
With phones, the 300 boxes are shared equally, 100 at each beach.
Full answer
(a) A: . B: . C: . Total paid: . (b) 100 boxes at each beach, so the price is $15.00 everywhere. Total: . (c) The fishermen together gain . Boats from beach A gain most: their fish now sell for $15.00, not $6.00. Buyers at beach C gain: they pay $15.00, not $24.00, and get more fish. Buyers at beach A lose: they now pay $15.00, not $6.00. Boats from beach C lose a little, because the price at their beach falls from $24.00 to $15.00. Check: 300 boxes are sold in both cases, so the extra $1 080 comes only from the fish reaching the beaches where they are wanted more. ✓
In 1945, the economist Friedrich Hayek asked how anyone can run an economy when the knowledge it needs is spread across millions of people. One person knows a mine has closed. Another knows a new factory needs a metal. Nobody knows everything. His answer used tin as the example. If tin becomes scarcer, for any reason, its price rises. Every user of tin then uses it more carefully, without needing to know why it became scarcer. The price carries the message. (Source: F. A. Hayek, "The Use of Knowledge in Society", American Economic Review, September 1945.)
The Kerala coast shows the other half of Hayek's idea. A price can only carry a message to people who can see it. Before the phones, each beach had its own price, and boats at sea could not see any of them. The phones did not catch more fish or create more buyers. They let the prices travel. That turned 15 small markets into one bigger market.
The next lectures look at each side of a market in turn: first the buyers, through demand, then the sellers, through supply.
- A market is any arrangement that lets buyers and sellers exchange something and agree a price, with or without a building or money.
- A voluntary trade happens only when the buyer's highest price is at least the seller's lowest price, and then both sides gain.
- The gain from trade comes from a good moving to someone who values it more, and the price decides how that gain is shared.
- A price is discovered by both sides searching, with sellers lowering prices when nobody buys and buyers pushing prices up when they compete.
- Competition limits how much any single seller can charge, because buyers can choose another seller.
- Markets need information, and the Kerala fish markets show that better information can reduce waste and help both sellers and buyers.