For all these transfers to occur, some preconditions regarding the buyer and seller should hold. The buyer must have enough money, know where the item is sold, and the seller should be willing to sell that item. In our theory we assume that all shopkeepers are willing to sell the items in their shops. In fact, shops exist to sell items. On the other hand, an ordinary seller who is not a shopkeeper should agree to sell the item before the money and item transfers take place.
Cash transfer is the most common one.
One assumption of the system is that money transfers of less than some fixed amount are made in cash. For an agent to use a check or a credit card he should have a bank or say, a Visa account. If an agent buys an item by writing a check, the balance of his bank account would decrease by the amount transferred. On the other hand, if a credit card is used, then the amount that the agent owes to the Visa account increases. Therefore, these accounts may have negative balances.