"When a Jet customer buys items that aren’t in its inventory or available from partner merchants, a Jet employee buys the items from another website and has them shipped directly to the customer."

http://www.wsj.com/articles/frenzy-around-shopping-site-jet-...

Edit: The context is that the article at the top of this thread is showing one way they are sort of "faking" partners. The concierge model I'm referencing is another. Hopefully, it's a temporary thing, because it's messy.