If invisible interfaces succeed – which I am not entirely sure they will – Alexa will function as a barrier of entry for independents sellers. Which is probably why Amazon is so focused on it. Google, Apple and Microsoft try to win in this field as well and they might be beneficial to the market, sending customers to the cheapest or most trustworthy seller.
Amazon on the other hand would be selling items from their own shop.
Of course, Google, Apple and Microsoft already have experience with setting up their own shops (Google Play, App Store, Microsoft Store) where they impose a sales tax on other sellers.