1. A computer-implemented method, said computer-implemented method comprising: providing 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; tracking a plurality of transactions performed on the item listings via the user device; assigning, for each transaction, a first numerical value to the keywords included in a selected item listing and a second numerical value to the keywords included in non-selected item listings, the first numerical value being determined based upon the sale format associated with the selected item listing; and building a desirability index using the numerical values assigned to the keywords for use in sorting item listings in a second search result identified in response to a second query, the desirability index including a desirability value for each of the keywords, the desirability value indicating an accumulative frequency of a corresponding keyword being selected throughout the transactions.