Market Making Simplified: Understanding Liquidity

A market maker posts both a bid and an ask — they'll buy from you at X and sell to you at Y. The spread between X and Y is their gross revenue. They're not betting on outcomes; they're betting on order flow. If they manage inventory correctly, they profit from the spread regardless of which way the market resolves.

On traditional markets this is done by sophisticated firms with enormous capital. On Polymarket it's done by automated market makers — smart contracts that provide liquidity algorithmically.

Polymarket uses a constant product market maker for many of its markets. The formula is simple: x * y = k, where x and y are the quantities of YES and NO tokens and k is a constant. When you buy YES, you reduce the YES supply and the price rises. When you sell, you increase supply and price falls.

This means: large trades get worse prices (slippage), thin markets have wider effective spreads, and very popular markets with lots of liquidity are much cheaper to trade. Understanding this helps you estimate your true execution cost before placing a trade.

A market showing 70% YES looks like a good trade if your estimate is 80%. But if the total liquidity is $5,000, your $500 position is going to move the price significantly. You enter at 70%, but your actual fill might be at 74% because your trade is moving the market against you.

This is slippage and it's the silent killer of small-market trading. Always check total liquidity before sizing a position. In practice, I keep my position size under 5% of total market liquidity to control slippage.

Most retail traders are always takers — they trade at market price and accept the spread. In some markets on Polymarket, you can post limit orders and effectively become a maker, earning the spread instead of paying it.

For patient traders with a thesis that doesn't require immediate execution, posting limit orders at your fair value estimate can improve returns meaningfully. You get filled when someone else is in a hurry. The tradeoff: you might not get filled if the market moves away from your limit before it hits.

The order book tells you the true liquidity story. A market showing a mid-price of 60% might have a bid at 55% and an ask at 65% — a 10 point spread that makes trading expensive. Or it might have bids at 59% and asks at 61% — liquid, cheap to trade.

Always open the order book before trading. The displayed price is the last trade or mid-market. The order book shows your actual execution price. Smart traders know this; most retail traders don't.

Liquidity concentrations tell you what sophisticated participants care about. A market with $500K in liquidity on a question about a specific political outcome means real money thinks this question matters. Thin markets on similar questions mean the sophisticated players aren't interested.

This is a meta-signal: if a market has high liquidity, something worth trading is happening. If it's thin despite the question seeming important, ask why smart money isn't there. Often the answer is that the resolution criteria are ambiguous or unfair — a good reason to avoid it.