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.
Like with many other things invented with good reasons this one will be a subject of many speculations and manipulations. In my opinion many of us are giving the best we can, who is that judge to say that we have enough quality to get merit? Until now I didn't give and I didn't get any merits, I don't have enough merits to reward many great comments, and in same time I think many of my comments pass unnoticed here, who will reward me? My English is far from perfect, I'm trying to elaborate my thoughts in the best possible way, but does that means anything to anyone?
People will trade with merits, here or somewhere else, doesn't matter. When everyone is allowed to be a judge of comment quality I doubt in fair system.