The moat
Scale in fulfillment, Prime subscription habit, third-party seller network effects, and AWS switching costs (data gravity, IAM, managed services). Brand as default internet store in the US.
Pain, risk, or retraining required to leave.
The product gets better as more people use it.
Patents, data, licenses, process knowledge, regulation.
Fixed costs spread over a volume rivals cannot match.
Structural ability to be the low-cost producer.
Pricing power from trust, habit, or identity.