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
Diving into one I think is detrimental to both sellers and customers, Ads. If you are not a huge brand this is a ~mandatory additional cost on top of FBA (or you won't show in results). As a customer it sucks because many of the top results are trash. Why bother having a decent product if you can just buy a ranking and some fake reviews? Amazon is pushing this so hard right now as it is a quickly growing revenue stream for them :(