IMO, selling merit regardless of the posting quality of the user is an unethical behavior and must be punished appropriately. So tagging them or giving neg trust is okay. This wrongdoing creates more chaos in the forum about merits, like for example a certain user is selling merits to another that can be considered to be a shitposter/spammer, the merit awarded will be then questioned by the community as to how that kind of post got merited while their own posts who they thought to be much more deserving got nothing. A long, repetitive and useless discussion about it will eventually occur.