A person can only give another person 50 merit points. Merit given is also public, so if someone were to give a spam/crappy post 50 merit, it could be caught by the community and the users given negative trust. Merit isn't very plentiful at the moment (I only got 200 points by being Legendary, which isn't enough to rank someone to Sr. Member), so it would also be very expensive. Hard to predict what will happen for sure, but I don't anticipate merit selling to really pick up.

Existing ranked accounts might go up in value though.