One way it seems they are using to prevent running out of merit is that they merit each other back and this way both parties increase in merit... I have seen a few threads were vigilant member of this forum are trying to track those people down and correlate the reciprocal meriting - often giving 50 merit points for posts that are by any standard not worth meriting at all.
These merits will ultimately run out, each time they do this they only get half back, eventually it will be at 0 and there will be nothing for them to trade.
Unless the person selling merit is a merit source.