I buy and read a lot of books. If you do as well, you might find this handy little tool called Booksprice useful. The tool allows you to enter a book title or ISBN and then get a report of the current price from dozens of sources including sources like eBay.
Here an example search using Duct Tape Marketing: The World’s Most Practical Small Business Marketing Guide
You can also use the service to compare prices on CDs, DVDs, and games.