Selling your books back to the bookstore might be the easiest and most convenient option (besides maybe burning them in a bonfire), but you won’t get much money for them, which doesn’t seem too fair for a poor college student.
A good way to maximize the amount of money you receive for your books is selling them on Amazon. It’s easy to get started with a seller account, and all the books you want to sell are most likely already listed and easy to find.
List the books you have, decide how much you want for them, and you’re all set. When someone buys your listing, you will be notified by email, and you have two days to send it off.
Amazon takes a percentage of the fee and you have to make a trip to the post office, but it’s worth it when you receive close to the amount you paid for the book in the first place.
Another option is taking your more expensive books to the same classes next semester and finding students who haven’t planned ahead and want to take advantage of a better price than the bookstore offers.
You don’t have to sell it for much less than it’s going for at the store for both of you to get a good deal.