You are right there is no rules in selling merit and creating a giveaway merit thread. It would probably just be much easier to buy an account in the first place.
Are you daft? Selling merits is an abuse of the merit system and selling/buying of merits will result in your account getting tagged by DT.
Merit selling has been going on for a while now, and it would be hard to track down those who do so. One of the ways however is to look at merited comments and see if it is actually worthy of a merit point. If its a rubbish post and it has been merited, then there should be suspicion about that. However if it is a good or great post, then it would be hard to track.
A good post getting merited is obviously the credit of the poster in the first opinion. Its not possible to track down every merit seller/buyer and since much of it can happen outside the forum it can take time to investigate as well.
However, it would be worth pointing out that for good/great posts, the people who buy merits, may feel neglected (as merit sources are scarce) and thus are pushed to buy merits, because they desperately need to rank up.
If they are really interested in taking part in discussions and interacting with other forum members (even though the toxicity in the forum is at brink) - they would not care about their rank.