Whether traded accounts are used for good or bad is down to your own personal opinion of what 'good' and 'bad' is. The overwhelming percentage of accounts sold are done so almost exclusively for signature campaigns and not used for nefarious purposes. Do you think that is a bad thing? I don't. Merely buying an account does not equals impersonation. Most are likely just honest or at least neutral people trying to make a bit of coin the easiest way they can. It may not be a crime but it may also not be dishonest either. Frowned upon, maybe, but not necessarily dishonest unless the person in control of the account does dishonest things with it. Can purchased/sold accounts be used deceptively and to scam? Of course, but so can accounts you can freely create yourself. Most people are not going to waste money trying to buy an account when they're likely to fail or get busted before you can successfully scam and then that account is ruined and worthless. 

I think the main rationale and reasoning for allowing accounts to be sold is because allowing them lets others know that the practice can and does go on and banning them may give people a false sense of security, not to mention banning their sale will not stop the behaviour from happening and only push it further underground and into obscurity. 

This forum also doesn't moderate or ban scammers, but that doesn't mean we allow or encourage them nor do we profit from it in any way from it.