Amazon has an enormous advantage being the biggest online retailer in the world. Now, most Americans do buy their phones at carrier stores. However Amazon currently sells on-contract phones too, so I imagine they will offer a seamless experience to buy new phones on or off contract.
All they really need to do is provide a solid phone that competes with today's flagships. If their Android skin sucks, it will lead to negative reviews. If they skimp on specs (I don't think they will) they will hear the negative feedback.
An Amazon smartphone is long overdue and I expect it to be, at the very least, a respectable offering.