All they can do is buy some merit points from members who are selling them. Such deals might be happening outside the forum and mods will not be able to catch all those deals. However, this will automatically decrease with time. After few months, the default merits of all the members will be spent and thus they will not be able to sell their merit points.
Then we will see merit coming from merit sources mainly and it is not possible to bribe merit sources to give merit points for money as they are established members.
Moderators can not know where they make their deals, but all evidence will be shown up in their merit histories. What do you think if low quality, one line threads received so many merits (10, 20, or 50 merit points)? It is really easy to find. Moderators will have solution for such merit abused cases. Don't worry, keep doing what you should and have to do to rank up (keep writing high-qualitied posts).