First, I think theymos should clarify whether it is against the forum rules to 'sell' merit or not. It should be. At the very least, it will discourage the more honest members, who don't want their account banned due to violation of the rules.
'sell' merit will be happen. this is what we should monitoring. people can make telegram channel for buy/sell merit, or other channel for this.
if sMerit not tradeable, you should delete and banned all account that made the thread, but you can control them in other forum such as telegram and slack