"In a barter exchange one good is traded directly for another. This form of exchange requires a double coincidence of wants, meaning that each trader has what the other trader wants and wants what the other has. Without a double coincidence of wants the exchange process can become exceedingly complex, requiring a great deal of resources to complete transactions, resources that cannot be used for production. In fact, inefficient barter trading is the primary reason that money was invented." - good explanation from AmosWeb