An agent device is connected to a user device for browsing a merchant site, for selling products online and being served up on the Internet, the merchant site, and a server device of a credit-card company. In the case where an instruction of purchasing a product browsed by the user device is output, the instruction is sent to the agent device. The agent device extracts a credit card number of a credit card held by a user having purchased the product, and inquires of the server device of the credit card company whether the product is to be purchased with the credit card. As an inquiry result, in the case where the product can be purchased online with the credit card, the agent device sends an instruction that the user purchases the product online with the credit card.