Best Shopping Cart for multiple vendors (managed billing) ?

I am looking for advice on choosing a shopping cart program that will allow multiple vendors to set up their own store on one central site – and then allow a visitor to buy from any one of them (or multiple vendors) and pay only one bill at checkout. So a visitor may cruise the site and find something in one store they like – add it to the shopping cart, and then go into another store and find something – add it to the shopping cart – and then go to checkout.
The system would also need to be…
Best Shopping Cart for multiple vendors (managed billing) ?

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