I don't see why the books couldn't be sold, either. There are plenty of used bookstores around, people selling books on ebay, facebook groups for selling books, etc. On the cactus and succulent FB page there are always people selling books.
It has nothing to do with copyright, you would need Dave's input but I would think anything plant related would be OK to sell. In the old co-op forum people sold all sorts of plants and garden supplies, Atlas gloves were always a good seller.