13. A system comprising one or more processors and one or more machine-readable media storing instructions for execution by the one or more processors, the instructions comprising: a predicted listing quality score module configured to determine predicted listing quality scores for a plurality of item listings at their respective times of listing when no historical data is available to assess performance of the item listings, each item listing comprising data representing one or more attributes of a product or service offered for sale; an observed listing quality score module configured to determine observed listing quality scores for the plurality of item listings from an assessment of their historical performance based on historical data; and a weight calculation module configured to determine, for each of the plurality of item listings, a weight of the observed listing quality score relative to a weight of the predicted listing quality score as used in a ranking score for the item listing that comprises a weighted sum of the predicted listing quality score and the listing quality observed score, the weight of the observed listing quality score increasing over time as a function of a time-based metric that increases as historical data for assessing the performance of the item listing becomes available.
13. A system comprising one or more processors and one or more machine-readable media storing instructions for execution by the one or more processors, the instructions comprising: a predicted listing quality score module configured to determine predicted listing quality scores for a plurality of item listings at their respective times of listing when no historical data is available to assess performance of the item listings, each item listing comprising data representing one or more attributes of a product or service offered for sale; an observed listing quality score module configured to determine observed listing quality scores for the plurality of item listings from an assessment of their historical performance based on historical data; and a weight calculation module configured to determine, for each of the plurality of item listings, a weight of the observed listing quality score relative to a weight of the predicted listing quality score as used in a ranking score for the item listing that comprises a weighted sum of the predicted listing quality score and the listing quality observed score, the weight of the observed listing quality score increasing over time as a function of a time-based metric that increases as historical data for assessing the performance of the item listing becomes available. 19. The system of claim 13 , wherein the observed listing quality score is based on at least one of a number of bids on the product or service of the item listing in an auction, a number of views of the item listing, or a number of watch lists associated with the item listing.