providing, by a commodity selling system, a first network address for a constituent member of a particular group and a second network address for a customer who is not a member of the particular group, said first network address being a first Uniform Resource Locator for the constituent member of the particular group to enter the commodity selling system and said second network address being a second Uniform Resource Locator for the customer who is not a member of the particular group to enter the commodity selling system;

judging whether a customer who has accessed a commodity selling system via the network is a constituent member of a particular group based on a network address of a shopping site accessed by the customer;

judging whether certification is to be performed based on the network address of the shopping site accessed by the customer;

certifying the customer by referring to a database storing constituent member information of the particular group, when the customer is judged to be a constituent member of the particular group;

selling an ordered commodity based on first sales conditions if the customer is certified as a constituent member of the particular group, upon receiving the order for the commodity from the customer to the first network address; and

selling an ordered commodity based on second sales conditions if the customer is not a constituent member of the particular group, upon receiving the order for the commodity from the customer to the second network address;

wherein both a first price as the first sales conditions and a second price as the second sales conditions are provided for the constituent member of the particular group through a first page identified by the first Uniform Resource Locator, the first price being lower than the second price, and the second price as the second sales conditions is provided for the customer who is not a constituent member of the particular group through a second page identified by the second Uniform Resource Locator.