It means that the stock was sold at market price instead of putting in a limit order at a certain price. Market makers take advantage of market orders by paying as little as possible for your shares by dropping the bid. As soon as your sell order is filled the bid pops back up. That is why some market makers will only show 10,000 shares at time, because that is all they are required to buy and if a market order shows up or too many shares come up for sale, they can drop the bid and purchase those shares for a lot less. Same thing happens on the ask when you want buy shares. It's a game that market makers play. Rule of thumb is that you always want to use limit orders when buying or selling. That is why you need Level 2 when placing orders.