I've gotten some amazing deals on Ebay. Last year I scored the Song School Latin complete set, CD, DVD, TM and Student manual (unused) for next to nothing. I have gotten all of my "big" books on Ebay, the complete versions of Pooh, Madeline, Curious George, Beatrix Potter, etc. for pennies. Kingfisher encyclopedia and Usborne for less than $10 each. Atlases, evan moor books are almost always cheaper, and a lot of homeschooling books. I search for title of book, author and then ISBN number. I've also gotten some great deals on "used" books from Amazon, but you can email the seller in both cases to be sure you're getting what you think you're getting. You have to compare prices, though. Sometimes the NEW books are cheaper on Rainbow Resource or CBD!
Also: if you aren't happy, SEND IT BACK and get your money back! I'm such a people pleaser that I have a hard time with this... but you'll be miserable the rest of the time you need to use it, remembering the bad deal you got. :(