Well, it becomes pretty obvious when you look at how SR actually operates: why don't sellers just post a list of products on a forum or wiki and buyers send them bitcoins? Because there would be no form of escrow or feedback tracking or interventions by third-parties or bonds required of sellers; with pseudonymous identities and irrevocable payments, it'd be an incredibly tempting lemon market. It's those additional features which turn it into a working marketplace.
Further reading: http://www.gwern.net/Silk%20Road#silk-road-as-a-marketplace