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.
I concur. Those who would abuse the merit system would risk their account to be punished. I don't really know what are the official rules regarding this but I may assume that it would lead to permanent ban in worst case. I do believe that theymos had anticipated this that's why he made it public when you give smerit.
Is the selling of trust a problem right now?
Good point.