I guess selling of merit is happening out of the forum or through PM, so such cases are going to be hard to prove ones. But in the case if such deals will be uncovered I believe those users will get negative trust as well.
Deals happening outside forum: If these are happening in online marketplaces, it can still be traced. It will require a lot of work on the part of staff though and I do not think they have that much free time. If you notice such a thing on any marketplace like Freelancer, you can bring it to the attention of mods.
Deals through PMS: Why are these hard to catch for the moderators ? Can't they check the pm of other members ? This is done on other forums I am part of .