To enable a seller to make conversation about a transaction and deliver an article while concealing personal information such as where to make contact and an address of a purchaser from the seller and to prevent one-sided annoying contact from the seller to the purchaser after the transaction.

A mediator system for managing personal information of both the seller and the purchaser and mediating conversations and a deliverer system for retrieving a delivery destination at the time of delivery of commodities are provided between a purchaser system and a seller system. The mediator system is provided with an electronic post office box generation program for issuing an electronic post office box for use in conversation between them with respect to each transaction, an electronic post office box program for transferring electronic mails arriving at the electronic post office box to a destination with a sender concealed excepting electronic mails of which the transfer directions and the numbers of times of return meet a condition of rejection objects, and an electronic post office box change program for allowing the purchaser to change the condition.