You pay money. A lot of it. And/or declare bankruptcy. The "person" you borrowed the stock from may well lose out, but we're talking about huge institutional i

You pay money. A lot of it. And/or declare bankruptcy. The "person" you borrowed the stock from may well lose out, but we're talking about huge institutional investors that lend stock thousands or millions of times a year, or even day, and the fees more than make up for the few losses they incur. This is not fundamentally different than any other loan.