2nd and Charles is a second-hand store that takes everything but clothes and housewares. I’ve sold some of my books back there and gotten what seems like pittance, but it was a better option than taking them to Goodwill (although a fair number of my books end up there).
I’ve also blessed members of our homeschool group with books I don’t need/want. The trouble with that is that if I’m objecting to content, probably they are too, as we all think along about the same lines when it comes to that.
I think MelissaB uses a homeschool classified site – I don’t know much about that, maybe she’ll chime in.