Glossary
Market order
Fill now against whatever is on the book. You are a taker: the price is the path through resting limits, not the screenshot mid.
A market order says fill me now against whatever is on the book. On a CLOB that means you are a taker: you lift asks (to buy) or hit bids (to sell) until your size is done or the order’s fill-or-kill / immediate-or-cancel constraint stops it.
There is no “mid price fill” unless the book is one tick wide and you are small. The fill price is the volume-weighted path through resting limits. On a $2k-deep politics name, a $25k take is a different trade from the 40¢ you saw on the screenshot. Polymarket UIs often wrap this as a marketable limit (a limit at a worst acceptable price) so you do not run through an empty ladder. That still takes liquidity; it just caps how far you walk.
Polydata never sends market orders. When the whale feed shows a print far from the prior mid, you are usually looking at a taker who paid the book — slippage, not a new oracle. Size a hypothetical win with the payout calculator; it assumes the price you type, not the path a real take would walk.