This is the reason why I quit buying anything from Amazon anymore if I can help it. They're not interested in fixing it.

What I can't understand is this: even if they comingle, they must know which seller owns which item. If they didn't, how would they pay the right seller once a particular item is sold? And yet, there seems to be no accountability at all.

They don't need to know which product belongs to which seller, they only need to know how many.

If someone clicks your listing and buys a product, they ship it from the commingled inventory and then pay you your share of the sale and deduct one from your inventory tally.

Then, if the item they sent ends up being counterfeit, whether from your own stock, from another seller, or even from Amazon's own stock, they will take it out on your seller account for selling counterfeits.

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.

Ah, it makes sense to do so but I'd imagine this practice would affect consumer confidence in Sold by Amazon options given time.

It already is. As you can see by everyone here talking about it, it's beginning to cost them sales. I imagine there will be a big campaign by Amazon in some years from now about them "trying" to fix this problem.

Not always true; only if that vendor uses co-mingling.

As a vendor you have to pay Amazon extra to avoid comingling - basically you have to get a new item number assigned and product you're selling all has to be tracked by that number.