Buying Used Books

• www.alldirect.com - this site is typically 40% off retail.  It has all kinds of books.  It is like Amazon, but not many people have heard of it.  Search by ISBN if possible. They don't send out your order as quickly as you may like.  Sometimes they will take 3 days before your order is put in the mail.
• www.amazon.com - I'm sure you already know this site.  It is worth consulting, but you will probably be able to beat the prices.
• www.christianbook.com - this site is good.  That have ALL the Christian books, but usually they are a little more expensive than alldirect.com.  Many times it is worth getting them hear because they have more Christian books in stock, they are put in the mail quickly, and they have plenty of choices of shipping methods.  If you want a set of commentaries, get them here.  CBD's sets are the most inexpensive.
• I no longer recommend www.discerningreader.com.
• www.scripturetruth.com - these guys don't have the best selection, but what they have are priced well.
• www.cvbbs.com - these guys have better books than most, and the prices are decent.
• www.gbibooks.com - this site has the best book recommendations.  Their prices can sometimes be good.

Buying Used Books

• www.abebooks.com - You can find some great deals here.  If you want to buy more than one book, remember that this site allows you to search many different bookstores.  You can use this site to search particular bookstores too.  For example, I use this site to search Pendulbury's Bookshop in London.  I have purchased more than 30 books from this one store. 
• www.gospelcom.net/kregel/usedbooks/ - Kregel publishes books that tend to be more conservative that other publishers (they could be more conservative though!).  They have a pretty good sized collection of used books (about 200,000).  It is best to look during their Christmas sale or their sale that takes place every summer.  Check the site for the details. 
• www.half.com - I have purchased about 10 books using this site, and I have found great deals.  I have noticed that they database can be messed up though.  Be careful.  I typed in an ISBN for an Old Testament Theology book.  When the book came up, it was attached to a different author.  I couldn't really tell if the book was the one I wanted or not.