Neither Apple nor Amazon sell iPhones on Amazon as far as I can tell. All the listings you see are third-party sellers. If you click on any of the listings, you'll see the list of sellers in the right-hand column under the checkout buttons.
For a long time now Amazon has been taking over that part of Ebay's market. (I think of it as the Ebay 'buy it now' market). Its a pretty sweet deal for them, skim off the tasty transactional cost cream off the top, leave the buyer and seller to hold all the liability.
> Looking on Amazon, I'm not seeing any iPhone 6 sellers other than Apple.
Look for "used" or "refurbished iPhone 5", there are many (many!) sellers like BREED and Vius. Most fulfill through Amazon, so this may be their way of trying to tackle this problem?
Looking on Amazon, I'm not seeing any iPhone 6 sellers other than Apple.