ROBIN: Here is my response to your question. I understand you built Fat Free Cart. That is pretty awesome. Here is the deal from a developer's perspective.
If you sign up for a free account on KickApps.com and start creating your own social network you might see what I see. Once you create your network anyone can sign up to become a member and thereby be permitted to upload videos, pics, and blog, tag, etc. Basically anyone can join your community and start contributing.
Now I imagine one of these social networks (or communities) can be used as an ecommerce site. Anyone wanting to sell their stuff can upload pics, videos, etc describing their products. Anyone wanting to buy the products can browse the site and click on any item to add to the cart.
So here is the trick. Every member of your community who is selling products has a unique id which is generated by KickApps. You must attach this id to each product that person is selling. You must then pass this id (and the product's own id, etc) to the shopping cart. When the order is completed, the community owner (you) must receive all this info and pass the order requests to the individual sellers for fulfillment.
I imagine one can use jQuery to create the cart's code on each product with the seller's id attached. I am probably thinking outside the box and really need to look into all these details further. If you can shed any light on this, please do so. Thank you.