Maximum number of posts per user in a specific forum

[ssfnl]

I was wondering if anyone has ever had the need to maximize the number of threads a user can start in a specific (sub)forum. We offer a free market place where our users can promote their used gear to sell. We would like to make a distinction between fair users and commercial users. For now, the only way we can think of to distinct both, is the number of threads they start. Would it be possible to limit the number of threads a member of a specific usergroup can start in this marketplace/channel?