You could achieve this with the multi-store version of KonaKart which however isn't free. In our next version coming out soon, we've introduced a multi-vendor mode which will allow vendors to maintain their own products. If a customer buys multiple products (from different artists) in a single order, the customer sees the shipping charges from the various artists and each artist receives an order containing only his own products. The payment goes to the main store belonging to the administrator who then has to redistribute it to the artists.