I used to volunteer in Oxfam Books and it's honestly one of the best things you can do unless you do know people who want the books obviously.
Everything is inspected and if for whatever it's not fit to be resold (big coffee stains, or missing pages etc) it's recycled. And then any profits go to help people in need. And we came across some genuinely rare things a few times. An edition of Shakespeare which was worth £400 or so from the 1700s was probably the most impressive - this like that get sold online.
Obviously if there's a different charity which means something more to you even better - I honestly think it's the best thing to do with old books these days