Sounds a lot like mastercoin. Why not just develop mastercoin?
http://www.mastercoin.org/https://bitcointalk.org/index.php?topic=265488.0You could simply use Ripple for this... Bitcoin is not really made to act as a marketplace from within, so you need an external network for trades anyways.
No you don't. Bitcoin had scripting capabilities built in from the start. It was always meant to do this.
very disappointing when i go to mastercoin.org
the 52 seconds introductory video does not explain what Mastercoin is, its just showing a logo splash screen, wth?