I have used Amazon in two ways:
1. Selling direct to someone else via Amazon market place. You'll probably make more money with this approach BUT they do take a cut and, importantly, they cap how much you can charge for postage, which can be an issue for very heavy books.
2. They also used to do a thing where if you had a book that was reasonably in demand, Amazon would buy it off you (they would set the price, which varied frequently according to supply and demand) and then sell it on for a profit. Your margin takes a hit but they pay the postage to send it to them. I managed to get rid of a 1000+ glossy page book that was 2 editions out of date via this method - I think I got about £13 for it. If I'd have posted it to someone myself it would have cost £5-10, but I'd have only been able to charge about £2.99 for p&p. I'm not sure if this option is still available though - you might have to do some digging.
I have also bought (but not sold) second hand textbooks from abebooks. Worth a look