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.
Merit sources do not generate unlimited merit; they have a very finite amount of merit that they can generate and it has a cool-down or regeneration period of 30-days. If you notice a merit source (or anyone) selling their merit it will be enacted upon very quickly; this will not stand with theymos or DT members and anybody purchasing that merit will get tagged, too.
Plus if you juggle your merit between accounts, this will also be noticed and expunged. So far I have seen a pretty solid zero-tolerance for merit abuse, and it is laying a good foundation for our new system. I have yet to hear/see of a case where merit has been successfully abused for any tangible benefit.
are there any rules and punishment 'bout it? can it cause an account to be banned?
Short answer, yes.