When someone sells, there's another investor stepping up to buy. When a stock crashes, that doesn't mean there is no one buying. It's just that the supply of stock being dumped by sellers has overwhelmed the buyers at that price. As a stock falls, the price triggers buying by investors who were waiting for a better price. Basically stocks can be bought at bid or ask, and sold at bid or ask as well.
“The secret of being a con man is being able to know what the mark wants, and how to make him think he's getting it.” ~ Ken Kesey