Create Drupal Commerce stores as a Service
This is known as the "marketplace" use case.

There is a 250-comment discussion on this in the Drupal Commerce issue queue:
http://drupal.org/node/990204
See http://drupal.org/node/990204#comment-5880498 for a fairly simple implementation plan.
Of course, it goes into details that your project might not need, but you can just ignore those.

Quoting the relevant part about the basics needed:


  1) An entity that represents the store. That can either be the user, a store entity clicked together through the ECK module, or a store entity provided by a module.
  
  2) Entityreference. Then a bunch of entityreference fields that point to the "store" entity (or user). One on products, one on product displays, one on orders.
  
  3) Form alters that hide the entityreference fields on product displays and products (or just use a "hidden" widget) and fill it in with the known value (you always know which store the user has. If not, give him a dropdown...).
  
  4) A custom submit handler for the add to cart form (replacing the existing submit handler) that adds a product to an appropriate order (since there's an order per store for each customer), and if the order doesn't exist yet, creates it and sets the value of the entityreference field.
  
  5) Custom view for cart/ that shows all active carts to a customer.
  
  6) Custom views filtered by the entityreference field that show the merchant the products and orders for his store.


Organic Groups fits neatly on top. You then need to figure out how you're handling payments, see the linked issue for various approaches.