1. A method in a computer system for matching item descriptions provided by merchants of items to item definitions, the items of multiple merchants to be sold through a web site provided by a web site provider, the method comprising: performing by one or more computers: providing, for each of a plurality of items, a unique item definition that is an authoritative definition of that item; receiving from a merchant an item description of an item that is to be offered for sale through the web site; automatically identifying item definitions as candidates for matching the received item description; automatically analyzing the candidate item definitions to determine whether the received item description matches a candidate item definition to a specified degree such that the item described by the received item description can be authoritatively determined; and when the item described by the received item description cannot be authoritatively determined by the automatic analysis, displaying an indication of attributes of the item description and candidate item definitions; receiving from a user a selection of one of the candidate item definitions whose attributes are displayed; and in response to receiving the selection from the user, storing a designation that the selected candidate item definition matches the item description; subsequent to storing the designation, receiving the item description from a merchant and, without dependence on user input, automatically using the stored designation as an authoritative determination of the item described by that received item description.