I've considered it, and I've bought some used books online, but I find the process pretty annoying. Online is a last resort after I checked all possible used book stores, when I do buy online it's generally half.com or amazon's used service. When I'm ready to get rid of books, I generally trade them in for credit towards new books - around here, a 2 for 1 trade with a 10% upcharge.
That being said, my sister-in-law just unloaded a huge collection of Barbara Cartland on ebay for $89 + shipping, great considering the used book store offered her $30. Special case tho - that was a full set of some series, 129 books. For single titles I don't think it's really worth it, considering the time and trips to the post office involved.