An order book is a list of buys (a.k.a "bids") and sells (a.k.a "asks") submitted by parties at various price points.
There are two types of orders that you can submit: market and limit orders.
Market orders are when you buy or sell shares from the pool of existing orders in the market. For example, Bob wants to buy 100 shares of NO. He looks at the order book and sees that there are "asks" for 75 shares @ $0.50 and 100 shares @ $0.55. If Bob places a market order for 100 shares, he will buy all of the 75 shares @ $0.50 and 25 shares @ $0.55.
Limit orders are when you buy or sell shares based on a specific maximum price and amount. Assuming the scenario is the same above, Bob wants to place an order for 100 shares of NO @ $0.51. If he places the order, he will buy all 75 shares @ $0.50, but the remaining 25 shares will not be "filled" and will appear on the order book instead.
There are currently no fees for our order books.
In order to trade on Polymarket questions, you must generate an API key. This key is unique to each account and will not require you to re-generate it if you keep using the same browser. However, you will need to re-generate the API key if you sign in with another browser or device.
Not all questions on Polymarket have order books. Some will still use the AMM structure.