Amazon is a great way to unload used books. But, you can also sell a variety of other items on Amazon as well. You could opt to start an Amazon FBA. You could source products on Aliexpress and drop-ship them from Amazon. That gives you a near-hands-free method of running an ecommerce business without much of the hassles of personally dealing with returns and customer service.
Hunt for under-priced used books that you can sell online. Download an app that reads ISBN numbers so you can scan the barcodes on books. This will pull up the book’s current price on Amazon so you can see if it’s worth trying to resell it. Then, visit used book stores, thrift stores, and garage sales to look for high value books. Post the books for sale online using sites like Amazon or Ebay.