We have two second hand bookstores in my area. One of them has changed hands a couple of times over the years, so I don't think they make a ton of money, although they are in a large, albeit older, retail space so they must do ok. The other keeps expanding in adjacent spaces, but seems to make a lot of their money selling book adjacent stuff (socks, bookmarks, pins, candles) instead of actual books. In fact the owner regularly stocks several of those free libraries around town and always has free books outside of her store.