Agent represents the agents involved in the buying and selling process.
Agents can be people or organizations.
Buyers and sellers are introduced to the system as members of this unit. Item represents the item involved in the buying and selling process. There is no limit on the number and kind of items that can exist in the system. Money holds the common attributes of money in different forms that an agent can have. It has two slots, holder and balance, and two subclasses, cash and account.