Buying used books on the internet - a guide
......The internet has unsurpassed selection. You find older books, as well as review copies for up to 50% off. The drawback to online buying is shipping; cost varies, usually $3 to $5 for a single book (rarely free). On the other hand there's no trip to the store and no sales tax. I've purchased hundreds of books with few problems. I formerly asked dealers to confirm availability and sent a check, but lately I've used credit cards (especially overseas) with no problem. Statistically your card number is more likely to be stolen locally than online. The place to start is Bookfinder (metasearch - 150 million books). AddAll and Usedbooksearch.net are less comprehensive but may contain items not found on bookfinder. It's fun to browse and useful for estimating book values. Amazon used books can now be found on bookfinder. Second party sellers (Amazon, Half Price), particularly individuals, are a little riskier.
......What about auction sites? Ebay's OK for mass market publications low in literary merit, i.e, Readers Digest condensed editions and old Playboys. Literary and academic types don't hang around ebay waiting for rare books to appear.
Amazon search - Amazon book reviews can help you decide whether a book is worth buying
Barnes & Noble search
Christian used books
Secret histories of used books