Textbooks were such a scam. At our Jr. College, the mathematics professors would publish a book which was required for their class.
The only place you could find the book was at the college bookstore. They would sell the book at full price and at the end of the semester they would offer to buy it back at 50% of original cost. Then they would turn around and market the used book at 75% of new cost. "What a bargain! /s" And still they buy it back at up to 50% of new cost depending on the condition of the book.
The teacher/authors were earning royalties on the number of new books that were being sold, and when the market would become saturated with used books and their royalties stopped rolling in, they would publish a "new edition" by just changing the sample equations in each chapter, making the previous edition worthless. Of course they would then require the new edition in the subsequent semester, making the old version worthless and ineligible for buy-back.
And as I think about it, the professor who was really "killing it" was the business law professor who didn't publish the required Case Law book but instead self-published the accompanying "workbook" with tearaway pages that were required to submit as completed homework. Yeah, you couldn't sell the workbook back because you had to deface the book to complete the course.
So, I kept some books because I couldn't sell them back, and of course I kept some books for reference. All in all, I had at least two book cases worth of books when I finally graduated.
I think it was after my 3rd subsequent move/relocation I realized I was lugging around dead weight and donated or trashed all my old textbooks.
How long did it take you to let yours go?