Selling books and textbooks you don't need is a great way to declutter and make some extra cash. There are quite a few good sites to buy and sell books, and BookScouter shows you their offers all in one place.
Once you enter the ISBN for your book, BookScouter generates a list that shows you a prediction of offers from various vendors, if they pay for shipping, and how they will pay you (check, Paypal, etc.). You can also check out each vendor's profile for reviews and ratings from people who have tried them. Surprisingly, the big names (Amazon, Chegg) aren't the ones to always have the highest offer so having this comparison list helps you get the most for your books. BookScouter does often have a buyback minimum (depending on the vendor you pick), but if you have several books to sell you shouldn't have too much trouble meeting it. They also have mobile apps for both Android and iOS, which you can try at the links below.
BookScouter (Free) [Google Play Store]
BookScouter (Free) [iTunes Store]