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.
Would be interesting to see how much tolerance is allowed for merit use ?
Repeated merit abuse should result in permanent ban in my opinion. Some people are not concerned about negative trust on their account simply because they are not using the account for signature campaign. Also, there are members buying accounts with negative trust here as well.