You know you can pay eBay to promote your listing. That is pretty much what they mean by ads in this case. Someone is paying to be the top spot or be promoted on search results.
I think Amazon has an ad network of sorts, so sellers bid for the sponsored search slot. In retailers who don't have such a platform, merchandisers take the call and boost/bury products in search results.
Ahh, that makes sense. It also explains why I see certain sellers ads everywhere from time to time.