Abstract: An online system for permitting users in a data network to manage inventories of their collectibles (including antiques) includes a database of collectibles, user inventory files, user information files, a database of needed items and items available for sale, and a server-based task for managing and searching the database of needed items and items available for sale. The database of collectibles includes object classes indexed by primary classes and subclasses, and user inventory files storing inventories of instances of the object classes, including instances that each user has, instances that each user would like to sell, and instances that each user would like to buy. Users may access their respective inventories by inspecting, editing, and adding the instances to the user inventories; obtaining valuations of instances in their inventories; and insuring instances in their inventories. The server-based task finds buy-sell opportunities between the users.