It is unnecessary to know exactly they exchanged merits and money for those merits.
Simply looking at post history, if the post history shows one user always post trash posts and another plus point is those merited posts are shitty ones too.
If the phenomeno occur from time to time, or one or two posts received 20 or 50 merits with one-line posts, trash contents, it is enough to say about merit abusements.
How can you report to moderator of the Merit is being sold outside the forum? I have seen this kind of offers as well in Facebook groups that is related in cryptocurrencies. Its hard to catch them because if its being sold outside the forum, you can't tell what username is being used by the seller unless someone is willing to go undercover and play as a buyer.
However, admin (theymos) even don't think people should tag merit abusers with red (negative trust) because admin think that they will run out of sMerits soon. It is obviously that the opinion of admin applied for small abusements only.
But I think if there would be a penalty to merit seller, merit buyer should be treated the same way too.
Yes, reputable users or user who have ability to earn significant amount of merits won't sell their sMerits for such $15 earnings because they can earn much more without such risk of merit abusements.
most of those who get them in large amounts are members with a good reputation which they can not risk.