Both the UI and UX of amazon's web store are terrible. The reason people buy from amazon is price, breadth of selection, and speed of delivery, not the wonderful shopping experience.

Trying to differentiate UI and UX in the context of a web app makes no sense. Both are two sides of the same coin. In your own comment, you couldn't even come to a conclusion as to where one ends and the other begins!

This is the hallmark of a buzzword. When a phrase like UX comes to describe everything and nothing at the same time it ceases to have meaning.