Banning bought accounts isn't the most challenges if refered with the incidents that had happened with the mentioned accounts. What matters most is how to get rid of the culprit who's responsible for selling of those accounts because when you get rid of those obvious bought accounts, the farmer selling the accounts will still create more accounts to be sold. That's if really it's succeeded in banning the available sold ones.
What I don't know is whose obligation is it that tracks bought accounts so they can always keep on hunting them to justify the forum scam free with the bought accounts.
Banning the buying and selling of accounts is a challenge. You're not just going to ban an account simply because you feel the writing pattern has changed or the person nows spam. Before you ban such accounts you have to be certain with proof that the account was sold and bought by someone else.
What if the person who bought the account is actually using it well? What if he makes posts that help the community and don't break any of the forum laws? What do you do then? Make an exception? I think there will be too much grey area if the buying and selling of accounts is illegal, that's why it's not illegal but discouraged.
Even in the real world, there are things that are not illegal, but not ethical.