Viaweb develops a search algorithm called Revenue Loop. Merchants bid a percentage of sales for traffic, and results are sorted by the bid multiplied by the average amount a user would buy. Graham:
Revenue Loop was the optimal sort for shopping search, in the sense that it sorted in order of how much money Yahoo would make from each link. But it wasn’t just optimal in that sense. Ranking search results by user behavior also makes search better. Users train the search: you can start out finding matches based on mere textual similarity, and as users buy more stuff the search results get better and better.