In at least some states, to sell more than XX cars per year, you must have a dealer license.
A company vehicle is really no different than a personal vehicle, at least not in this context. They own a truck (titled to them), they can sell a truck via private sale. They don't get any sort of dealer perks (such as not being listed in the history as an owner), they are just selling their property.
Why can't they just copy Tesla's model? Or does Amazon operating a storefront on behalf of the manufacturer preclude that?