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What does it mean “sell on ask” , “sell on bid” in stocks?
It's good to ask this question, because this is one of the fundamental dichotomies in market microstructure. At any time T for each product on a (typical) exchange there are two well-defined prices: At time T there is literally no person in the market who wants to sell below the ask, so all the people who are waiting to buy at the bid (or below) could very well be waiting there forever.  There's simply no guarantee that any seller will ever want to part with their product for a lesser price than they think it's worth. So if you want to buy the product at time T you have a tough choice to make: you get in line at the bid price, where there's no guarantee that your request will ever be filled, and you might never get your hands on the product you decide that owning the product right now is more valuable to you than (ask - bid) * quantity, so you tell the exchange that you're willing to buy at the ask price, and the exchange matches you with whichever seller is first in line Now, if you're in the market for the long term, the above choice is completely immaterial to you.  Who cares if you pay $10.00 * 1000 shares or $10.01 * 1000 shares when you plan to sell 30 years from now at $200 (or $200.01)? But if you're a day trader or anyone else with a very short time horizon, then this choice is extremely important: if the price is about to go up several cents and you got in line at the bid (and never got filled) then you missed out on some profit if you "cross the spread" to buy at the ask and then the price doesn't go up (or worse, goes down), you're screwed.  In order to get out of the position you'll have to cross the spread again and sell at at most the bid, meaning you've now paid the spread twice (plus transaction fees and regulatory fees) for nothing. (All of the above also applies in reverse for selling at the ask versus selling at the bid, but most people like to learn in terms of buying rather than selling.)