Our kids never told us how they got their textbooks. Somehow S would make a profit every year after his 1st semester of college (and his freshman year of HS). I believe he made friends with EE kids in the year above him and would buy their used books inexpensively and resell them or buy international copies of the correct version of the textbook and then resell them.
I bought D her books for CC and her 1st term of college; no idea how she acquired them thereafter–her brother may have helped her, as he is very resourceful or she may have gotten them from others in her Cinema school.