Hi Marisa419!

You might try J & J's Books and Coffee on Calle Espiritu Santo, 47. I doubt they'd buy-back the textbooks but maybe the novels. They're a vast 2nd-hand book English-language book store where I've bought loads of books. They also offer free Wi-Fi and is a kind of Yank/Brit/Aussie hang-out and also organize social events.

Good luck! Saludos, MadridMan