Abstract: When a customer buys a product at a store or by way of storeless sales system 5 (web server), that product is automatically registered with data management system 4 as an item owned by the customer. Storeless sales system 5, either at regular intervals or on demand, updates resale prices for items owned by the customer within data management system 4 to most-recent values therefor, and displays these on a customer website which can be accessed by customer terminal 7. When a customer at customer terminal 7 views, from that customer website, a catalog page for a product he wishes to purchase, storeless sales system 5 furthermore displays that catalog such that a difference in price between the selling price of the product the customer wishes to purchase and the resale price of an item owned by that customer are calculated thereon.