No one knows exactly when people first started trading with one another—or how. We do know that metal coins have been used to buy and sell things for at least 4000 years. From horses and handcarts to ships, trucks, and airplanes, the need to trade goods has spurred on innovations in transportation for just as long. Today, though, it's all change: many of us are now buying and selling with a new form of commerce that involves neither money nor transportation—at least not in the traditional sense