OP, there's not a lot that's going to be done, unfortunately.  Theymos doesn't really care if people sell accounts, farm accounts, or any of that.  If he did, there are many things that could be done, the most obvious of which would be to ban account sales on the forum.  So I think your suggestion will fall on deaf ears.

Even tagging account buyers & sellers is of limited use, since most people create sock accounts for those purposes.  But I still get a big grin on my face when I find and tag a sold account that's being used to spam.  If you find one that's obviously been sold, drop me a PM.