Among the best online resellers of books are Half.com, Alibris, AbeBooks, and Amazon Marketplace. Between these four, you can find most any book you're looking for — and at a discount. Each of these sites has ample information about the book's condition so you won't exactly be buying "blind".
An added benefit of resellers like this is that, since there are multiple individuals trying to sell the same book, there is price competition. Most sellers just want to get rid of the book, so they aggressively price their books to move. Which is great for the buyer, like you, to swoop in and get a bargain. Getting a book for 50% off, using this method, is not very hard to achieve, though sometimes shipping kills the deal.