I sold 221 cards on Amazon in September, out of 1,664 total on COMC, which is about 13% of the total number of cards. The average selling price of a card sold through Amazon was 163% higher than the average selling price of a card sold on COMC, so Amazon was about 29% of my COMC sales.
In my experience, the cards that typically sell on Amazon through COMC are much different than the cards that sell on COMC. There is a much heavier emphasis on non-sports, gaming, movie, tv, celebrity, superhero, rock n roll, pop culture type stuff. There is much less emphasis on condition. The types of cards that tend to sell on Amazon used to just sit gathering dust on COMC before they started cross-posting to Amazon, and most of them wouldn't be worth submitting to COMC, except with the expectation that they might sell on Amazon.
My impression is that many of the Amazon customers have never set foot in a card shop and that a lot of them might have not even been looking for cards at all. They might have purchased a 1955 card as a birthday present for somebody born in the year 1955. Or they might have been doing an internet search for Superman, and stumbled upon a trading card with Superman on it.