The source is this post in this thread. The question seems philosophical to me: someone can pay a merit source, and the merit source can give merit. But it's not allowed for a source to sell his merits.

Although I didn't find that answer all over the forum but this is still the best reply for this question. I guess?

Consequences for the selling merit could be very bad for them.
Lutpin says: "It isn't allowed for sources to sell their merit, a punishment could go from being booted as source all the way up to getting your account banned, depending on how far you stepped out of line."