The invention provides a sales system in a communication network whereby a mediator in the network permits a purchaser the opportunity to purchase his desired commodity for a lowest possible price and a seller the opportunity to sell the same commodity as much as possible. The mediator can receive bids from both of a purchase wisher and a sales wisher to announce the bidding results with no limits in period and receive changes in the bidding conditions until transaction conclusion, thus changing conditions based on the transaction conclusion state to provide lowest possible purchase prices and highest possible sales prices.