To protect our users from potential price volatility, Alpaca automatically converts market orders into limit orders using a 2% price collar. Collaring helps cushion against sudden significant price movements and prevents overspending the available funds in your account. We use the last ask price for buy orders and last bid price for sell orders at the time of order submission to determine the limit price. Meaning that a market buy order placed at a 9.90 bid x 10.00 ask will have limit of 10.20 for the order. Please note, for notional, market orders this can potentially lead to fills less than the requested notional amount.