Perhaps many buy and sell merit points in secret, I think it is very difficult to get merit points, most people will probably take a shortcut, maybe they have multiple accounts to send each other..
I agree, it is actually not that easy to track merit sells, especially if they are not made in big amounts and arranged somehow outside the forum or in local boards. However, I guess the system will work fine eventually. It is just that some improvements and explanations should be made.
Maybe it would be a good idea to set a limit of 5 merits one can give to one post, for example. At the same time, some encouraging actions should be made, because I have this feeling that most of people ignore the merits they can give to others and even when they agree with a person they don't merit him/her.