The only difference between a Merit source and someone else is that a Merit source isn't allowed to sell Merit.
As far as I know nobody is allowed to sell merits in forum, not just merit sources, maybe this is not written in forum rules but everyone knows this is a not endorsed.
It happened before several times, they got caught and they deserved negative feedback for sure.
I think it's a bit like scamming: the forum doesn't take action against it, but the community does.