With Promoted Listings, eBay does not simply choose the the two listings with the highest ad rate to boost up the search results:
In addition to looking at a seller’s set ad rate, other factors are also considered, such as relevancy and how well an item is selling at the time the ad rate is set.
This is backed up by feedback from sellers, that the items which benefit most from Promoted Listings are those which already have a high sell-through rate.
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New items, with no sales history at all, can also benefit. But existing items with a poor sell-through rate are unlikely to do well.
This might feel a little unfair and circular. Sellers often want to use advertising to “fix” their under-performing listings, and don’t see the point in paying to promote listings that are already doing well.
But it might be worth paying to promote listings with a good conversion rate, but low traffic, and turn that good performance into a lot more sales.