- traders willing to gamble on high-risk situations -- a chance to buy more shares. If their investment decisions are correct, speculators can make a greater profit, but if they are misjudge the market, they can suffer bigger losses. The Federal Reserve Board (frequently called"the Fed"), the U.S. government's central bank, sets the minimum margin requirements specifying how much cash investors must put down when they buy stock. The Fed can vary margins. If it wishes to stimulate the market, it can set low margins. If it sees a need to curb speculative enthusiasm, it sets high margins. In some years, the Fed has required a full 100 percent payment, but for much of the time during the last decades of the 20th century, it left the margin rate at 50 percent. Selling Short. Another group of speculators are known as "short sellers." They expect the price of a particular stock to fall, so they sell shares borrowed from their broker, hoping to profit by replacing the stocks later with shares purchased on the open market at a lower price. While this approach offers an opportunity for gains in a bear market, it is one of the riskiest ways to trade stocks. If a short seller guesses wrong, the price of stock he or she has sold short may rise sharply, hitting the investor with large losses.