Check out BookStealer.com. It’s a great online tool for book collectors. I have been using it almost daily and have found many bargain books.
To use it, you need to get a login (FREE and EASY). You set up a search which includes search keywords, a minimum price to be considered worth searching for, and a percentage that is how much the book must drop to be worth looking at. Run the search. The first time you run the search, BookStealer will search Amazon, and remember any books over the minimum amount. The next time you login and run the search, BookStealer will search Amazon, and look for any books that have dropped by the percentage amount you entered, or more. This is very useful when looking for expensive out-of-print books. Sometimes you get lucky, and a marketplace seller will list a valuable book at a fraction of it’s normal selling price. BookStealer will help you find these underpriced books. You may not find a bargain every day, but if you set up good searches, and use it regularly, you are bound to find some great book deals! BookStealer does all the work, you reap the benefits.
Give it a shot!