Where will I get the most money selling my textbooks?

Consider renting them out on https://www.shelflet.com/, you could potentially recoup the entire cost of your books...and still sell them, if you choose to do so. Its a better alternative to selling at a deep discount to your local bookstore or on Amazon.

You'll get the most money selling your textbooks to other students at your school. All of the ways you mentioned are the most convenient way to get the least money for your books. If you want to get the most money, you have to give up the convenience of selling your books to a third party for immediate cash. Sorry, that's the way it works. Using a middle-man always costs more than a direct sale from one student to another. I took some cash to the first day or two of each class. It was kinda fun when a student showed up to the start of class to offer a book for sale. I would already have a price in mind for a cash sale, so I could negotiate on the spot for the best price. The other students were shocked and amazed when they saw me buy a book right in front of them for less than half the price they paid, while the seller still got a better deal in return than the bookstore or online buyback vendors. I taught them a bit about the power of cash and negotiation in a way that most had never seen before, which was rather useful in my business classes. A piece of plastic is no good compared to cash in this situation.

I would check what the bookstore is offering as a buy-back price, and if it doesn't seem reasonable, just post a list of the books you're selling on Facebook. Most universities have a buy/sell Facebook group...

Don't sell it back to the school.

Sell them on Craigslist or Ebay.

you can sell textbooks there's a store somehwhere here in california lol you can sell or buy them but idk i forgot what its called well, sell what you want to sell