This might be a possible solution to think off.
What it would be like if shitposting Hero, Full, Sr who never received a single merit in this 8 months gets demoted

Like this one?
r1a2y3m4 hadn't receive any merit since it was implemented.
I'd say the next step should be for everybody (all Ranks!) to lose 1 Merit every x days. It could be as low as one per year, or maybe once every 3 months. That should demotivate users from buying one Merit somewhere.
Any normal poster won't be influenced much by losing 1 Merit once in a while.
But it's a good start already!
Yes, this will do the trick. It's almost 8 months since merit system got implemented. It's already a long time and if a ranked user hadn't manage to get a single merit, this shows his/her inactivity. This will also affect account sellers who holds high ranked inactive accounts.