Merit was made to determine one's post quality.
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.
But selling-buying is actually happening or just exchange between friends and multiple accounts. On Local boards you may see such suggestions and other things about merit. I really do not know where the forum's moderators look for if such merit abusing stuff is happening like daily.