20. An apparatus comprising: at least one hardware processor to run a search engine, the search engine configured to provide a user device with a first search result including one or more item listings in response to a first query received from the user device, each item listing including a plurality of keywords and being associated with a sale format; and a tracking engine, executed by the at least one hardware processor, to track a plurality of transactions performed on the item listings via the user device, and to assign, for each transaction, a first numerical value to one or more keywords included in a selected item listing and a second numerical value to one or more keywords included in non-selected item listings, the first numerical value being determined based upon the sale format associated with the selected item listing, the assigning comprising determining the first numerical value based at least in part on whether the sale format associated with the selected item listing is a fixed price sale or a non-fixed price sale, the tracking engine configured to build a desirability index using one or more numerical values including the first and second numerical values, the desirability index including a desirability value for each keyword, the desirability value being determined based on one or more first numerical values assigned to the keyword, and the desirability value indicating an accumulative frequency of a corresponding keyword being selected throughout the plurality of transactions; wherein the desirability index is accessed to sort item listings in a second search result identified in response to a second query.
21. The apparatus of claim 20 , wherein the first numerical value is a positive value upon determination of the sale format being an auction sale.