This is a very good idea. I have always thought is was common sense to not post on a subject one knows nothing about but apparently it's beyond the understanding of a fair number of people.
I would also like to add that I abhor the postwhoring going on in the for sale sections, particularly the private car for sale section. I don't understand why people have to give their opinions on the worth of a seller's car. We're all big boys and girls here and should be able to decide if we want to spend a certain sum for a certain car. Personally the only time I can posting to for sale threads are friendly bumps for cars I like, a question about a really nice mod or part on the seller's car or if I see something drastically wrong that would be misleading (which hasn't happened yet).