i report many times users that sell bitcointalk but i see they keep sell with safe trust .
i think users must give red trust for illegal things or scam which is both not found when sell bitcointalk accounts
you report those users to the mods... It's not because a "regular" user thinks that selling accounts is untrustworthy (thus deserving red trust) that the mod will feel the same... AFAIK, mod's can delete, lock or edit posts and they're in direct contact with the forum admins when they feel somebody needs to get banned.
Since selling accounts is not against the forum rules, mods cannot delete or edit the user's posts, nor can they recommand a ban to the forum admins.
I personally don't have a strong opinion on account sales (don't really like the practice of account selling, but i don't feel strong enough to leave negative trust to account sellers), but if you personally think account sellers are untrustworthy, nothing is stopping you from giving them negative trust yourself...
So, to summarise
=> clicking on the "report" button will send a request to the mods. Mods have to moderate infractions against the forum's rules. Since account selling isn't an infraction, they probably do nothing at all
=> a lot of people feel that selling accounts is one of the reasons the post quality got so low, and so many scammers are active on this forum, so they personally think an account seller is untrustworthy. That's why they give negative trust. A mod might also give negative trust, but that's under his own name, and not under his "jurisdiction" as a forum moderator.