Thryft
Started by students from the National University of Singapore, Thryft is a second-hand online bookstore where you can trade in old books for credits to purchase other books on its marketplace. If your shelf requires you to free up over 20 books, you can trade in or donate em' hardcovers and paperbacks with Thryft. However, they have to be in good condition, which means no water damage, no broken or torn spine, and no missing or torn pages, as they will be finding their second (or third) home. All you have to do is snap a photo of the books for Thryft, and it'll come to collect.