Sales Score (aka Popularity Score) is an item level metric which is used to keep track of the recent sales of a given listing. This is carried over when you re-list an item. This metric is never shown to the seller in raw form but instead is used to influence search ranking. An important thing to note is that only "recent" sales (along with other best match parameters) are considered for ranking. Here are some must-see articles that discusses recent sales history:
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Recent Sales History should not be confused with the "Number of Items Sold" which is visible on the view item page on eBay ("Other Item info" section on the page). It is a running total of items sold for a given Item ID on eBay. It does not carry forward when an item is re-listed and it is not taken into account for determining search ranking.