Goodwill won't even take textbooks. I think the top of my single-stream
recycling can says no textbooks. Shame they were so expensive at the time
that we had to purchase them.

A decent second-hand bookstore should buy them. At least, I've bought
several from second-hand sources to keep as references and they must
have got them from someone. Of course, if you happen to have a copy
of Euclid's Elements Vol. I and Vol. III (Books I, II, X, XI, XII, and
XIII), I'd be happy to take it off your hands to complete the set.
The local library doesn't even have the buggers anymore.
Aut inveniam viam aut faciam