I don't think it can be prevented, but that doesn't mean account sales should be tolerated.  Thus I voted for banning account sales, even if it's not entirely enforceable.  The reasons why people shouldn't trade accounts has been written innumerable times, and I don't feel like making the argument again here.  Ultimately it enables people to scam or to earn more money by shitposting in campaigns, and there's really nothing good that comes from it.