Yes, anyone can make a website, anyone can accept payments, and anyone can promote their site and reach an audience. (and it'll take time and money; people are free to use another service or build it themselves - isn't this what a free market is all about?)

Amazon is the e-commerce juggernaut; Amazon.com is the 7th most popular website in the world. (4th in the U.S.)

The dominant player in a market sets the price. (apple & google play take 30%, uber & lyft take 20%, think amex/porsche/jetblue/nike...)