Tim's advice is right on the money. While I have found books on Ebay, as a collector, I'm usually searching for a specific book. And not only that, I'm looking for it to be in certain condition, and most probably for as close to a first edition as I can get. Sometimes I'm looking for an edition by a specific publisher or for an edition published in a particular location, so all of that information is important to me.
When I'm on these type searches, I go to AbeBooks, Alibris or some similar site. In reference to books there priced at a couple hundred dollars versus not being worth a few dollars on Ebay, sometimes (often times) those on Ebay have no idea of a book's worth. I have certainly known of individuals searching for a particular book, or books printed during a certain time period (18th century books) or on a specific subject, that paid way more than $250, and considered the purchase price a bargain.
But as Tim said, in order to get those kinds of prices, you have to do your due diligence. As a collector, I want to know everything possible about the book before I lay down that kind of money, especially because I can't see it "in person" or hold it in my hands for a personal inspection. The more information you can provide, the better chance you have to sell your items for top dollar.