I am confused. First you said you wanted to require people to post in your intro forum before being allowed to post in your sales forum, but then you said the offered solution was too strict, even though that's what you asked for?
It sounds like just want to prevent people from posting in your sales forum if they have not posted anywhere else. Is that correct? If that's what you want, you can do it with user groups and promotions.
If that's not what you want, you might try being a bit more specific.