I'd say rent them to people on Chegg or something. Then every semester that that book is used, you'll always have another $50-$60 in your pocket.
I stopped selling my books back, though. After selling back my calculus book, I told myself I wasn't gonna do that again. That was such a useful book I don't have anymore.
Matter fact, I actually want to buy BACK some of the books I rented. But I'm glad I still kept the ones I did.
My stats books.
My real analysis books.
My advanced engineering mathematics book (ODE, PDE, numerical methods, and complex analysis)