Monitoring Price Fluctuations of Book Trade-In Values on Amazon

I am planning to finish school soon and I would like to shed some weight before moving on. I have collected a fair number of books that I will likely never use again and it would be nice to get some money for them. Sites like Amazon and eBay let you sell your book to other customers, but Amazon will also buy some of your books directly (Trade-Ins), saving you the hassle of waiting for a buyer.