I recently bought an item from Amazon Marketplace. The seller sent me a hand-written confirmation dispatch email, which happened also to forward me the email Amazon sent them. From it, it was clear that Amazon’s cut for Marketplace sales, at least of books, is 15% of the total price, or 20% of the price before shipping. (With only one data point, I can’t tell which.)
Just thinking about the sort of volumes they must do, and the tiny amount of actual computing resources it takes to make such a sale once the system is set up, I suspect they are making what is technically known as a boatload of money.