I'm always selling textbooks and study guides, but not antiquarian books that much. I actually find them all the time when elderly people purge their places or pass on (or even libraries). There are TONS of them outside, but I only take a few. They're often hard to value, not in "fine" condition, and likely not worth too much. First editions can have value, but it's a bit laborious to check each one. Most of the ones I take are simply gifted to friends or kept as a nice piece for my own place.