I wouldn't mind used books being sold so long as the author/publisher got a cut. Selling the used items could easily put some of them out of business.
You are honest and some others are. But some people would forget they had sold a book and sell it again. Some would sell it again and again because they need the money until they couldn't sell it. Some people would pirate the books, having never bought a single copy, and find a way to sell it. All of these copies that don't pay the author/publisher/producer hurt the author's potential income stream.
Is there a moral or ethical reason to not allow reselling? I don't know. If laws forbid it, then it would be unethical and illegal. If the laws allow it, then it would only be a kindness if people didn't resell--not really an ethical or moral issue. The arts have often been partially supported by generous patrons because there often isn't enough commercial interest to support art. This is as it ever was.