they “borrow” shares from the unwitting true shareholders. When a brokerage firm opens an account for a new customer, it is usually a “margin” account—one that allows the investor to buy stock on margin, or by borrowing against the investor’s stock. This is done although most investors never use the margin feature and are unaware that they have that sort of account. The brokers do it because they can “rent” the stock in a margin account for a substantial fee—sometimes as much as 30% interest for a stock in short supply. Needless to say, the real shareholders get none of this tidy profit. Worse, they can be seriously harmed by the practice. They bought the stock because they believed in the company and wanted to see its business thrive, not dive. Their shares are being used to bet against their own interests.
A broker can't automatically give you access to margin trading. You have to request it and then fill out an application as well to see if you qualify. Not all investors qualify for margin trading. Brokers typically have strict requirements to ensure they do not extend capital to individuals who are incapable of generating returns that will repay the loaned funds.