However, Theymos has said tagging buyers/sellers is a legitimate use of the trust system (he made a post in reply to one of mine, but damned if I can find it).
But I can 
I'm wondering whether you specifically disapprove of account dealers being tagged--not necessarily your opinion on the matter, but whether you'd consider that an inappropriate use of the trust system.
Since some people view account sales as fundamentally untrustworthy, I think it's an appropriate use.
Forum has three hack waves and sold accounts are mostly hacked accounts since many years ago. With those sold accounts, you can easily find strange things as proof of account ownership changes.
With younger accounts, after those hack events, you will have more difficulty to prove account ownership changes.