So then how do they decide which of the comingled sellers got the sale? Round Robin? Random?

The customer still ordered via the listing of a specific vendor, even if Amazon shipped from co-mingled stock.

Not necessarily, some items have only one listing for all sellers and you click the buy button that has the prime logo. Who are you buying from in this case as far as Amazon is concerned for their inventory accounting?

It's always been clear to me which specific vendor I'm buying from, it lists it right under the product. I can click on new or used in order to see other vendors and select one of them if I prefer. Based on my experience, the vendor defaults to Amazon if they have the product themselves, otherwise it is likely based on whoever they make the most money on. That is not a straightforward computation - they might make more profit on a more expensive listing, but then fewer people would buy it. I'm sure they use some proprietary ML algorithm to decide which vendor to show, but its always been very clear to me which vendor I'm purchasing from, both on the item page and also in the cart.

Yes, but that’s not I was talking about in my original post, at least. I was discussing the fact that even if you explicitly choose “Amazon.com” from that list of vendors, Amazon itself is shipping goods from other vendors and making it impossible for you to know that, which can lead to receiving counterfeit goods. There is zero indication, anywhere, that it’s being shipped from anyone else’s stock in these cases.

I don't think this is the case. If there is only one listing for all sellers, it must be from amazon stock.

Maybe, that would make sense. In any case, I quit playing the amazon lottery altogether personally.