2.1. Barter

Because the opportunity cost of a good cannot be fixed, it is impossible to know the proper exchange ratios for barter. This problem is overcome in the market by using commodities that have marketable characteristics, such as transportability, durability, and a recognized economic value, to serve as a medium of exchange. Prices of goods do not strictly follow the quantity theory of money.

From Part 2: Money and Interest. Narrated by Millian Quinteros.