One thing Amazon is constrained on is on price across its catalog.  With competition beginning to match it on price, it will put a cap on it, this may not be a problem yet, given that consumers will give Amazon the sale because of force of habit and convenience.  However, I would be surprised if Amazon can continue to make razor thin margins at infinitum. At some point in time the market will require Amazon to prove it can grow into the valuation awarded the company.  I know, I know, it seems the market does not care about that, nor has it for the past 10 plus years.