That is a non trivial feature, if that is critical to your business model shopify is currently not a seamless fit though there are work arounds, and there are several apps and services which do faciliate it:
Or for the advanced create a channel though I have yet to see someone use shopify itself as the nexus
Me too! My main product is original paintings, and up until now, I've been hesitant to put many of my originals up in my online store, as it's been geared to prints made by Shopify's partner fulfillers (apps). I don't like the idea (and the expense) of warehousing my original works, and I'd appreciate being encouraged to sell my originals through Shopify.