Amazon has merely a slice of ecommerce. They have a commanding lead only in their type of ecommerce: being the new age Walmart of the Web. That leaves room for the other 80% of the market (in Walmart's case, they couldn't dominate most categories eg: dollar stores, Target, Costco, Home Depot, paint stores, convenience stores + gasoline, banking, pharmacy, clothing, tens of thousands of boutiques, grocery stores, Macy's & upscale, jewelry, video games, electronics, liquor stores, and on and on).

You have to do what Amazon isn't doing. Ultimately as ecommerce doubles in size in the US over the next ten years, Amazon is not going to get most of that. It's a staggering opportunity in terms of scale.

They couldn't beat eBay at auctions. They couldn't beat Craigslist at classifieds. They can't beat Priceline or AirBnB. They couldn't beat Google at search. They couldn't beat Apple at phones. They won't own online restaurant ordering. They will probably fail at trying to own services (Angie's List, legal, health, whatever). Amazon is going to lose in most things not directly tied to what they do today.

Amazon's inability to dominate the other 80% of ecommerce, is your opportunity.