I could imagine a user using Google just to circumvent the category hierarchy of a site like Ebay, e.g. searching for "Ebay washing machines". If this user clicks an ad on the result page, it is super painful for Ebay.
It may have to do with search boxes being built in browsers already. Type and search, instead of going to ebay and then type-search. eBay may be bookmarked but it's still not as easy to pull up as searching from a browser's window.