All "Market Buy" orders are actually submitted as "Limit Buy" orders with a 5% collar added on top of the price to protect you from adverse price changes and to prevent your account from going into overdraft. This ensures the order functions as a "Market Buy" order but offers added protection in case the price increases dramatically.
For example, if you submit a "Market Buy" order to purchase a stock that has a price of $100, we will submit this as a "Limit Buy" order with a limit price of $105. This means that your order will only get executed if the market price is $105 or less. If the market price is higher than $105, the order will remain open until the price falls below $105.
If you're seeing a pending market buy order, this would mean that the limit price (i.e. the last quoted price + the 5% collar) was not met yet. Your order will remain pending until the stock price drops and your limit price is met and if not, it will be ultimately cancelled at the end of the day.
If you wish to avoid a similar situation moving forward, you can cancel the existing order (a market buy order that's been pending) and choose to submit a new order using a more recent quote.
Market Sell orders do not have a collar and will always be executed at the best available price, regardless of how the price may have changed.
All fractional orders are filled at the end of the trading day. If you place an order at the end of the day or after hours, it’ll be executed at the end of the next trading day. You can cancel your order up until an hour before market close.