Platforms such as Google and Bing offer companies the option to "buy" words. This means their websites appear more prominently if a person searches for a particular term.
The eBay study found that most people who clicked through as a result of this service were loyal customers who would have come to the site anyway.
As the article goes on to suggest though, these results will be heavily skewed by the fact Ebay is such a well known brand to begin with, it would already show high in the listings. Smaller advertisers would probably see more benefit.