If you don't want people to post rubbish in the market place don't let them. Don't allow members to reply to threads but let them start them. Then implement some auction system whereby a member can up the current bid.

I like the idea on contribution in terms of an article to be allowed to post in the market place. There is something that everyone can contribute, it doesn't have to be about design or coding. It could include business, the internet, personal organisation or something :P