IMO it's not an unfair one, but due to being one of basically 3 big ones and def the biggest and most convinient one, everything Amazon-owned(their own stuff i.e shipment, etc) got pretty low-quality, meanwhile people fighting for everything even when selling on Amazon(just look at stuff like Amazon Listing Optimization like https://webycorp.com/listing-optimization/ - people are paying so much to get ahead of the competition...)

I don't think it's a monopoly, but if it was it'd be the most fair one of all time