Theymos said not to worry from the point of running out sMerits not from the point of open buying/selling IMO.
This statement is consistent with the Forum's stance that they don't ban the promotion of fraudulent or questionable activity. The proper course of action is to post a scam accusation thread in the Reputation or Scam Accusation board.
You could also post negative trust for anyone selling or buying merit in such public fashion it can't be questioned. But this approach reeks of witch hunting.