Metadata service that supports user-to-user sales via third party web pages

An electronic marketplace system embodies various inventions for assisting users in listing products for sale, locating the listings for a product, and performing related actions. A user wishing to sell a product can browse to an existing description of the product in an electronic catalog and then select an option to create a corresponding listing. To create the listing for the selected product, the user preferably specifies the product's condition and a selling price. Others may view and make purchases from such listings while browsing the catalog. In one embodiment which is part of a web page metadata service, the World Wide Web serves as the electronic catalog such that users may create, view and make purchases from product listings during viewing of third party web pages that describe the associated products. The system may also provide an option for buyers to preorder products from unspecified sellers.