Hi Criag,
this seems to be an excellant idea, but I was just wondering if the basic purpose of the site would be sidelined. Though, people like me would be quite interested in what others in the field have to offer, it would be subjected to a lot of ads from other commercial people. Also, you will strain your self too much if you and Steve would sit down to scrutinize each and every ad (like you do for posts). Is it really worth it?
Instead, let the users, along with some interesting stuff, put up what ever they have to sell in their home pages up in this site.

Warm Regards,
Harsha