Buying on Margin. Americans buy many things on credit, and stocks are no exception. Investors who qualify can buy "on margin," making a stock purchase by paying 50 percent down and getting a loan from their brokers for the remainder. If the price of stock bought on margin rises, these investors can sell the stock, repay their brokers the borrowed amount plus interest and commissions, and still make a profit. If the price goes down, however, brokers issue "margin calls," forcing the investors to pay additional money into their accounts so that their loans still equal no more than half of the value of the stock. If an owner cannot produce cash, the broker can sell some of the stock -- at the investor's loss -- to cover the debt. Buying stock on margin is one kind of leveraged trading. It gives speculators -61