It is the fact, but there are other two facts:
1) Shitposters, and higher-ranked users who ranked up to high positions due to the old ranking system before merit system launched will hardly to be accepted by strict campaign's managers. Especially shitty higher rank users that could not earn even one merits, or could not satisfy the minimum required merits (I usually see strict campaigns require ten self-earn merits over last 120 days to join).
- 1- Most sold account are bought to participate in signature campaigns
- 2- Most users buying high ranking account tend to be shitposting the hell out of it
Yeap, and they tend to publish shitty posts.
- 3- Most bought accounts don't earny any merit, or just a few
Agreed with you. So, why not make demotions aim at higher rank users like the wave of demotion on Junior Member.
The forum will turn into chaotic situation, when that massive higher rank users complained about this.

- 5- It is still fairly easy to get 10 merit to be able to wear a signature, even without adding much value to the forum
I don't think that the idea is realistic.
If a new system, for example Spam Control System, implemented, it will lead to massively debate in the forum, and it might be abused.
Be able to flag or "tag" any account for shitposting. This could be done through the Trust system or by a separate function.