12. The system of claim 11 , wherein the tracking engine is to track user activity including a user selection of a listing, a user bid on a listing, a user purchase of a listing, or addition of the listing to a user watch list.
11. The system of claim 7 , further comprising a relevance engine to derive the measure of customer demand for the keyword in the title of the listing by utilizing data from a tracking engine configured to track user activity associated with listings including the keyword in the title.