I kept all of my books, even the history, English, psych, etc. stuff. I kept them because if they were being used the next semester, the college bookstore was going to pay me 1/5 of what I paid and sell them to the next student for 75% of the new price. If you're going to sell them, make it a private deal via cork boards at the college dorms and classrooms. You can make more and the buyer will pay less.
The calculus book by Thomas was big and broad enough that it was used for three separate math courses at the University of Missouri - Rolla: differential calculus, integral calculus and analytical geometry. So that book ($9.95 in 1967, a real budget killer and probably $80 today) was at least worth the money.
I've never looked at the old books much, mostly the chemistry and physics texts. They just occupy shelf space and remind me of good times.
Dean