I’ve sold, worked with real (not dropship) businesses that sell and a customer.

As a seller going through multiple channels if you are selling books for example - Amazon basically pays - many bookstores slow pay, even distributors can get way behind and require lots of phone time. The trade is that it’s all on amazons terms. Same thing with fulfillment- many sellers complaining have never actually run a warehouse and done pick / ship. I don’t doubt complaints by sellers are valid, I just wish Amazon dialed down the marketplace side because it’s too full of junk for me