Market orders are not the same as marketable orders. Market orders are instructions to buy or sell regardless of price. They're dangerous because in a sudden pr

Market orders are not the same as marketable orders. Market orders are instructions to buy or sell regardless of price. They're dangerous because in a sudden price move you may get a bad price (e.g., in a flash crash you may sell a $100 stock for 1 cent). A marketable order is any order for which there's a resting order on the opposite side that can execute against it. Market orders are by definition marketable (unless there are no orders at all), but limit orders can also be marketable (e.g., if someone is bidding $10, and you send a limit order to sell at $9.99, that's marketable). An exchange can function perfectly well without market orders (you just always have to specify a price), but not without marketable ones.