Why a market order fills at an unexpected price: the order book, market versus limit orders, slippage, makers and takers, tick sizes and a checklist.
The current price on screen is the last price that traded. The price you can actually buy at and the price you can sell at are separate, with a gap between them. The distance between the highest price buyers have posted and the lowest price sellers have posted is the spread. Buying and selling immediately starts you down by that difference.
An order book displays how many orders are stacked at each price level. A large stack means that much trading is needed to move past it. Resting orders can be cancelled at any time, though, so a stack that looked like a wall sometimes disappears as price approaches. The book is a photograph of the current state, not a promise.
The two differ in what they guarantee and what they give up. Which suits better depends on the situation.
A market order consumes the best-priced orders first. When your size exceeds what sits at that level, it moves to the next, and the average fill price shifts accordingly. That is slippage. It is barely noticeable in heavily traded assets and widens sharply in thin ones or during violent moves.
Posting an order to the book and adding liquidity makes you a maker; taking an existing order for an immediate fill makes you a taker. Many exchanges charge makers a lower fee, to reward thickening the book. For frequent trading, that difference accumulates.
Each exchange sets a tick size — the smallest step by which a price can move, in won or cents. For low-priced coins, one tick is a larger share of the price, so a single step can be several percent. With such coins, buying and selling immediately starts you at a meaningful loss from the one-tick gap alone. There is also a minimum order amount; smaller orders won't go through, and a tiny leftover balance may be impossible to sell. Checking the exchange's tick size and minimum order amount before ordering cuts down on unexpected fills.
When prices move sharply, things behave differently. Orders on the book are cancelled en masse, the spread suddenly widens, and market orders fill by skipping several levels at once. A stop order set to limit losses, if it is sent as a market order once its trigger price is hit, can fill far worse than the price you chose. Requests can also pile up on exchange servers, delaying orders or screen updates. So trading in a fast market carries more cost and uncertainty than usual. No order type guarantees the outcome you want in that situation.
Order mistakes usually happen when rushing. Before ordering, confirm the coin name and trading pair (won market or another coin's market), and check that the quantity and price fields have the right number of digits. For a limit order, make sure the price isn't far from the current price; for a market order, check that the book has enough volume to absorb your order. Common mistakes include pressing buy instead of sell, entering an amount in the quantity field, and forgetting an unfilled limit order that later fills at a price you no longer want. Clear out resting orders regularly. This guide explains order mechanics and recommends no trades.
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