The simplest form of trade is barter; I trade goods that I have and you want for goods that you have and I want. This raises a problem. I must find a trading partner who has what I want and wants what I have: what economists call a double coincidence of wants. In a simple society in which there are only a few goods being traded, this may not be a serious problem; but in a complicated society such as ours, it is. If I want to buy a car, I first look in the classified ads to find someone who is selling the kind of car I want, then call him up and ask him if he wants to be taught economics in exchange for his car. This drastically reduces the number of potential trading partners.