to be available to borrow, shortable, but you know nothing about the other end of the transaction. It's just like buying and selling long. When you end the trade, you're basically returning the shares you borrowed to your broker. If you're interested in shorting small caps or even penny stocks, only a few brokerage firms will have any shortable inventory of those shares.

A broker is willing to lend the shares because they charge you interest as long as the trade is open.