Drop Shipping/Consignment

Does anyone know of an extension or module that users can upload products to a store that get combined with the store's existing inventory?

(I have seen the marketplace extensions where users have their own profile and own inventory.)

I am looking for something where you wouldn't know the original store is a marketplace, you see products and stock as normal with no knowledge it's a marketplace.

e.g. users could upload identical products & sizes but they will get sold (under one product with shared stock) on a first come first served basis?

You can have a single store, the have different user groups which can upload products in the admin. You will need modifications though so the user groups can only see their own products and sales or just add to an existing inventory, front-end works like a normal opencart site.