My understanding is that you're allowed to sell things however you want, but inside the app you have to use Apple's purchase methods. You can't use your own, and you can't put directions in the app to go to a website or something else to purchase content. So in the app you have to do what Amazon does and never show it by button or a "how to buy" button.

And just for extra clarity, Apples policies only apply to digital goods. If you buy a real world service or physical item through an app Apple does not take a cut, but they also don't handle the payment. That's why Amazon doesn't have to pay Apple 30% when you buy a new TV from them.