The final user.
Personally, I've never used Amazon before seeing a friend ordering a fairly huge amount of stuff from there. Prior to that, I was a not-so-obsessed-from-ecommerce eBay user, where I placed an order maybe every 4-5 months. I've started using Amazon because of its returning policy, which is better that eBay's, but by doing so, i've also increased the amount of stuff bought on the platform, because of time. Getting older and having more responsibilities, you've to save some time, then Amazon comes to rescue when it comes to generic stuff.
I guess now it's really late to blame Amazon for the monopoly, as well as Microsoft did in 90s with PCs and Google is doing nowadays with Android+Chrome.
Sure, there are alternatives, but the final decision on who gets the monopoly is the user