I get what you're saying, but communities don't get built that way. They get built by members actively engaging in discussion and by doing so creating a zeitgeist of sorts.
The trading section is a perk for those who have proven they are willing to put in some time to make this place great.
It's really not that difficult to reach member level if you take an active interest in the forum, and I don't see how being granted a shortcut to it based on your reputation in another forum helps build this one.
Editing to note that 15 not sequential days is just 30 sequential days - a month - no great task to ask of a user.