This is nothing new. Account buyers and sellers have been using new accounts to avoid red trust for a very long time.
Only few foolish people give red trust for account buying and selling. Account buying and selling is allowed on forum rules. Hence, I think who give red trust are breaking forum rules. They are doing misuse of their DT power.
I know account selling is prohibited in the forum and they will be red-tagged if they caught.
Who told this to you. Theymos told you privately? He never told it is not allow and also never told sellers/buyers will deserve Negative trust. I noticed maximum negative trust is giving only two DT member.
Account buying and selling can just happen off forums, telegram maybe or even other forum sites like ogusers.
The only thing you can do is ban the accounts that get bought, and that's hard to find unless it's a shit poster talking on a legendary account that was speaking perfect english.
Then you should create a new forum and make your own rules. theymos is not a child so he know what is better. theymos allows to buy/sell accounts