I think I can define the generic :)
Multiple virtual locations ('markets') with multiple vendors in each market AND any vendor may be listed in more than one of the markets.
Customers see one market at a time.
For an non zen example, visit http://harvesttomarket.com, select New Hampshire and look at the Weare, Webster Ridge Locally Grown and the Contoocook markets.
You will see that several vendors are in more than one of these markets. Vendors do not sell directly - thus some have a 'market' containing one vendor.
The payment system allows a customer to order from multivendors at one market in one "order", each vendor gets paid their share. The only thing not well separated is orders - each vendor sees an order that contains all vendor's sold items. There are many rough edges but the basic workflow is there.
This example supports actual physical markets but they are only delivery points. Nothing else makes them different from any other cart except the grouped aspects.
I am interested in seeing what this will take to do. I fear it is a lot. The question is whether it is better to start from scratch.