Double Coincidence of Wants
Voluntary exchange can be beneficial. In order for exchange to occur, both parties must have something that the other one wants. Trade requires someone to give something up in exchange for something they believe is better. If the other person doesn't have something that you want then you won't be willing to engage in trade.
Show students the following scene from Moneyball where Billy Beane has trouble exchanging with the General Manager of the Cleveland Indians. Both GMs go back and forth trying to identify potential players that the other one may be interested in obtaining. The two of them are never able to find exactly the right mutually beneficial trade: