I know how to solve (chinese) farmer hackers and mastery book issues

This would cause inflation in everything that isn't mastery books or scrolls, I think.

How so? With all the mesos disappearing from the economy by buying them from NPC shops, there would be less mesos circulating the economy. Yes there's less stuff to spend the mesos on since there wouldn't be people selling scrolls and/or mastery books anymore, but there is still the fact there would be overall less mesos in the economy. Inflation is not that likely; prices of all other items would either remain stable or even deflate if enough mesos get taken out of the game with the NPC scrolls/book shops.