Besides swapping used books with your friends or as a PaperBack Swap member, you can also sell them online in Facebook's marketplace, on eBay, AbeBooks or via the OfferUp app. Offline options include local bookstores, consignment shops, yard sales and book-buying markets. If you would rather donate your old books instead of selling them, the most convenient options are used bookstores, local libraries and reputable charities like Salvation Army and Goodwill.