Many genealogists use ebay to make a variety of purchaes. Keep in mind when buying books that you may be bidding on a book that is still in print. Find out what the publisher is charging for the book before you bid. I have seen several occasions where a bidder ends up paying MORE for the book on ebay than they would have had they purchased the book directly from the publisher.
I ususally try and find the book for sale somewhere else online and then use that to base my bid--assuming of course that the book is still in print.
Save your extra genealogy money for copies ;-)