Account describes the bank account of an agent. Account describes the credit card account of an agent. It has one additional slot, limit, that indicates how much credit an agent has. Shop represents the shops in which the buying and selling takes place. The kinds and the number of shops in the system are not limited.
Transfer represents the transfers performed in buying and selling.
Transfer represents the transfer of items in buying and selling. Transfer represents the money transfer from buyer to seller in a buying and selling process. This unit is also used when the buyer should receive change from the seller. Buy represents a demand for buying an item. When an agent wants to buy an item, this unit is used to hold the information of the buying and selling process.