The land of Mesopotamia did not have a lot of natural resources, or at least they did not have the ones in demand during that time period. So, to get the items they needed the Mesopotamians had to trade.
In the southern part of Mesopotamia, docks were built along the sides of the rivers so that ships could easily dock and unload their trade goods. The merchants traded food, clothing, jewelry, wine and other goods between the cities.
Sometimes a caravan would arrive from the north or east. The arrival of a trade caravan or trading ship was a time of celebration.
To buy or trade these goods, the ancient Mesopotamians used a system of barter. For example, in exchange for six chairs, you might give someone two goats and a bag of dates. You had to work out an agreement and make a deal for the things you bought. Tokens were made of clay. They were designed in unique ways. Each shape had a meaning. One chape might mean a fancy chair. Another might mean a bag of dates. Prior to loading the chairs onboard a ship for delivery, token would be placed in a clay ball. The clay ball was then sealed and witnessed and signed with cylinder seals. If you had made six chairs for someone, you would put six chair tokens inside the clay ball. When the person who bought the chairs received their delivery, they opened the clay ball and counted the tokens to make sure everything had arrived as ordered. Deliveries and the opening of the clay ball were witnessed so there was never a problem.
They used barley for local trade. You had to borrow barley from a barley banker. The banker charged very high interest.
Because barley was heavy, they used lead, copper, bronze, tin, silver and gold to "buy" things away from their local area.
It was a very cumbersome system, whether you used barley, clay balls and tokens, or copper and gold. Many historians believe that this system of barter is what triggered the invention of the first written language, cuneiform. It was much easier to write a contract that was witnessed, and sign it with your cylinder seal.