If someone buys and sells merit it means that he abuses the system.
I can guess that Merit trading is forbidden and those who try to trade Merit will probably get their thread to be deleted.
Merit also very helpful as it gives users the option to thank other users for their posts.
An ability that wasn't available before.
I have a limited number of merits so I keep them for really useful posts. I may like different posts but still I will not waste my merits for them because I may need them in the future. So this system doesn't work for me and I see no sense in it. Well, except for the fact it's harder now to move to the next rank, so the competition in signature campaigns is lower - but again that is not significant for me.