Just make it illegal to sell accounts,
You mean like it's illegal to sell heroin and absolutely no one anywhere in the world sells or buys drugs anymore?
Exactly! Doesn't make sense to punish a few because of the few causing a nuisance.
You were probably trying to say that it doesn't make sense to punish everyone because of the few causing the problems.
Just disable signature if someone doesn't get X merit in last 120 days, it's more fair towards non-spammer and old members.
This is a good suggestion, but it would have to be done on a case by case basis. Even then, it will create controversy. Admins can manually disable signatures anyways, and have been doing that for members who get pardoned for plagiarism.
Maybe renaming "illegal" to "ban-able offense" is more appropriate words
I would say we have bigger problems than that. This is a forum where thieves and scammers can stay with only a tag on their accounts after it's been proven they stole and swindled.
The forum doesn't moderate scams is the official response. How would an account buyer be a bigger treat to the community and deserve a ban when a thief doesn't?