Searching google, you'll find a handful of companies that actually buy your books - companies that you can ship your books to, and they will send you payment when they receive them. Some names include Booksrun, eCampus, Buyback101, and more. However, the price each of these companies is willing to pay can vary by a lot. If you have a stack of books to sell, it's impractical to go to each company site and get an offer for each book. If you're selling ten books and ten potential sites willing to buy them, that's over one hundred price lookups. Introducing textbook buyback comparison sites - these websites take the pain out of finding the highest offer. Just enter an ISBN, and they will check offers from multiple sites so you get the best deal. Here are a few options to choose from.
BookTrapper.com - Yes, the website you are on right now compares buyback offers from top vendors. Additionally, it lets you post books for sale at your school instead of in those Facebook textbook exchange groups that don't really work that well anyway. We also have a tool that allows you to estimate whether it will be better to buy or rent a textbook. That tool can be found here.
BookFinder - One of the original book buyback comparison sites, BookFinder has been around since 1997. Their interface is a bit dated, and it's not mobile friendly, but it gets the job done. The site was aquired by AbeBooks - a buyback vendor - in 2005, and in 2006, AbeBooks was bought by Amazon. So BookFinder is owned by Amazon.com.
BigWords - This site does other things besides compare book prices - they have a whole assortment of product categories you can search, including mobile electronics. They claim you can compare prices for just about anything.
BookScouter - One of the top buyback sites, BookScouter does a really good job of aggregating prices from as many vendors as possible. Type in an isbn, and you'll get probably the widest variety of offers. However, there are only a few big vendors, and those few are mostly going to show up in the top three in any price search.
A Word on Minimum Buyback
Many of the vendors listed on these websites have a minimum amount they are willing to buy and ship for free. It wouldn't make sense to them economically otherwise. If you have a few books to ship off to those vendors willing to pay the most, by all means do that. But if you only have a limited number of books, you'll need to go with the vendor that gives you a good overall amount and includes free shipping. Don't worry about small fluctuations in offers because you may not even be able to ship to some vendors if you don't have the minimum quota.