Prevolio
MarketsClassroom · multi-user

Double Auction

Run a live trading pit: buyers and sellers post bids and asks, and the market discovers a price with no one in charge.

You’ll learn: See how a competitive market converges to equilibrium and why that outcome is efficient.

Teacher guide

Students will learn to

  • See how a market settles on a price even though no one is in charge of setting it.
  • Understand competitive equilibrium and why transaction prices converge toward it.
  • Connect each buy/sell decision to surplus, and the whole market to efficiency.

Before the session — ask

  • If you were selling something, how would you decide the lowest price you'd accept?
  • Can a room full of buyers and sellers agree on a price with no one in charge?

After the session — discuss

  • Did the transaction prices end up near the competitive equilibrium? Why might that happen?
  • Who captured the most surplus, and was the overall outcome efficient?
  • How would the result change if there were many more buyers than sellers?

Timing

  • Briefing & role cards3–5 min
  • Open trading8–10 min
  • Debrief & discussion8–10 min

Run it with your class

Students join with a code or QR — no account needed. Teachers start a session from the dashboard.

Join with a codeFor teachers

What is a double auction?

A double auction is a market where buyers and sellers trade at the same time, each posting what they're willing to pay or accept, and deals happen whenever a bid meets an ask. It's the mechanism behind stock exchanges and commodity pits — a live trading floor with no one setting the price. What's remarkable is how well it works. Even though every trader knows only their own valuation and acts in their own interest, the flurry of bids and asks quickly converges on the competitive equilibrium price: the single price at which the quantity buyers want equals the quantity sellers offer. This is price discovery — the market surfacing a price that no central authority computed. And the outcome is efficient: the buyers who value the good most and the sellers who can produce it most cheaply are the ones who end up trading, so the total surplus (the combined gains of buyers and sellers) is as large as possible, with no mutually beneficial trades left unmade. The double auction is the classic demonstration that decentralized markets, under competition, can reach the efficient outcome that theory predicts — a striking piece of self-organization.

How the simulation shows it

You run a live trading pit where buyers and sellers post bids and asks and trades clear whenever they meet. Trading yourself shows price discovery happen in real time: with no one in charge, the market quickly converges on the equilibrium price, and the traders who value or supply the good most efficiently are the ones who deal.

Common misconceptions

Frequently asked questions

What is a double auction?
A market where buyers and sellers simultaneously post bids and asks, and a trade happens whenever a bid and ask meet — the mechanism behind stock exchanges and commodity markets.
What is price discovery?
The process by which a market surfaces a price through the interaction of many bids and asks, rather than any central authority setting it. A double auction converges on the equilibrium price this way.
Why does the market reach the competitive equilibrium?
Because trading pushes prices toward the level where the quantity buyers want equals the quantity sellers offer. The give-and-take of bids and asks converges there quickly, even with private information.
Why is the outcome efficient?
Because the buyers who value the good most and the sellers who can supply it most cheaply are the ones who trade, so total surplus is maximised and no mutually beneficial trade is missed.
What is surplus?
The combined gains from trade: a buyer's surplus is how much their value exceeds the price, a seller's is how much the price exceeds their cost. The efficient outcome makes the total as large as possible.

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