Recently I have become more interested in buying used books online, and had success ordering from https://www.betterworldbooks.com/ and https://www.thriftbooks.com/.
I'll check this out if I am looking for something new.
http://blog.archive.org/2019/11/06/for-the-love-of-literacy-...
It's a very good used book meta-search engine that sorts by price and condition, and includes specific notes from the merchant in the results page. I went through computer science/engineering in college using it, and one semester managed to get one-off editions of all of my books for less than $20 total.
Using it also ends up getting you acquainted with a lot of online used booksellers. It also includes Amazon, but you start to notice that a lot of the specific copies of books on Amazon are actually also being sold by the same sellers on friendlier or their own platforms, and when they're on Amazon they're often marked up to cover the Amazon tax.
sold books as cheap as like $2 and with free shipping