Commingling helps consumers (they get products quicker) and has very little downside. Typically people who are saying it does don't realize that Amazon still tracks which seller sold any particular unit regardless of which order Amazon shipped it for. If A sells a unit and Amazon ships B's inventory to fulfill it faster, Amazon still knows to get A in trouble if there's an issue.

The problem with commingling is counterfeits. Counterfeits are often not easily detected by the customer without significant effort.

That's an issue with or without commingling.

It isn't. Without commingling I know for a fact that if I buy from Amazon I'm going to get a genuine product (I don't really think Amazon acquires their stock through grey channels). With commingling, I no longer the guarantee of knowing which seller's stock I'm receiving.

The customer loses valuable information and decision power with commingling, especially important in these bazaar-type situations.

>I don't really think Amazon acquires their stock through grey channels

There are plenty of vendors selling to Amazon that aren't the brand owner. Look up Apple vs Mobile Star, they sued a company that was selling (allegedly) fake products to Amazon as a vendor that showed up as "sold by Amazon", nothing to do with commingling.

And the fraction of customers that care about choosing which seller to buy from is low, compared to those who care about price and speed of shipping.

Amazon turns off commingling on high risk products, fwiw.

Isn't that the point though? It was B's inventory that was fake so why should A get in trouble?

Sorry, was a typo. Can't edit now but the point is they can get the right person in trouble.