They can have bank accounts and credit cards. There are several other ways. However, these three are the most common ones and our theory covers only these. Cash can be used physically. In order to use the money in a bank account, checks are used.
Agents can use as much money as they have in their bank accounts.
In the case of credit cards, agents have a card and a related account with a limit. They can spend an amount of money not exceeding this limit. When they spend some money using a credit card, they owe that money to the account and should pay it back later. Each agent in our system has an amount of money that he can spend in a buying and selling process. This amount is the sum of an agent's whole money, either in cash, bank account, or credit.