To provide a trade-in guarantee system or the like allowing contribution to expansion of a reuse market.

A terminal installed inside a store acquires a customer identification number of a customer, and inquires a trade-in money amount from a host computer. The host computer accesses a database, acquires the trade-in money amount, and transmits the acquired trade-in money amount to the terminal. The terminal performs settlement of a commodity to be newly purchased by a money amount obtained by subtracting the trade-in money amount from a price of the commodity to be newly purchased by the customer. A store terminal acquires a guarantee trade-in rate from the host computer, and presents it to the customer.