I don't see how amazon is a monopoly. Sure, it has huge value due to the breadth of products and the number of consumers. But each consumer can shop at a different (or multiple) other stores and each seller can sell at multiple stores. No one is really locked in. Everyone is there because it is popular and good enough.

The only real "monopolistic" piece is the economy of scale. I almost never need to pay shipping on amazon because I can bundle multiple, even unrelated, items together. This potentially lowers their shipping cost and is one of the few things that would make be buy something from Amazon instead of a different retailer outside of convenience, price or security.