But in principle, eBay can defend its market dominance more than google.
Say there is eBay, and eBayCompetitor. eBay starts off with all the customers. As a single seller or buyer, I can chose to move to eBayCompetitor but it does me no good - if there are no sellers or buyers at eBayCompetitor except me, the site is useless.
On the other hand, if everybody is at Google and someone starts GoogleCompetitor, I can leave Google and GoogleCompetitor would work just fine for me, because it doesn't matter whether anybody else uses GoogleCompetitor. You could end up with some kinds of semi-steady state like in the browser wars.
The problem in practice with search engines is that I am starting to get the idea that the "market" likes having a single search engine to worry about/buy ads to/manipulate ranks for. So it is possible than in reality there is some pressure against GoogleCompetitor gaining traction.