> This is a bit off topic, but why are used books so expensive on abebooks, thriftbooks, amazon so expensive compared to booksales, etc?

A mix of enforcement, laws and good old market capture.

20 years ago I could bring a suitcase full of brand new books - fiction or not, from India, no questions asked. These are functionally the same as those that cost 50x more in the U.S. - just that they are black and white, paperback and use cheaper recycled paper that will yellow in 20 years and become brittle in 30. I cannot bring them with me anymore. The books clearly say they are not for export especially in the U.S., the print for this uses some kind of ink that shows up on XRays clearly and TSA enforces it.

Similarly, used books in the U.S. are repurchased by bookstores to be sold at a profit.

From what I see students doing - they sell or exchange books over FB Marketplace or school lists or in person.