True, but that's useless to the customer, who has no idea of whether commingling happens or not.

The other day I found myself needing printer toner. Amazon has a much better price, even when they sell it themselves, than local stores. But then I see that along with the "sold and shipped by amazon", which is still quite cheap, there are dozens of even cheaper options, some of which are fulfilled by Amazon anyway. So how do I know I am buying a new, original cartridge, an honest, quality refurb, or something that might print badly, or even damage my printer? Even the sold by Amazon merchandise, which I'd hope is original, could be comingled with a refilled knock off that might be indistinguishable out of the box, but will be very different in operations.

For something like this, as a consumer, I absolutely need to know the provenance of what I am purchasing, and the chance that there is commingling means I cannot buy something like this from Amazon, as I won't even be able to tell for a while.

You should make it clear that this is on the merchant's side. Consumers are not allowed to choose where their purchases come from.

So interestingly - I’m an amazon shopper who’s unhappy occasionally. Where to I flee to? I want to buy a new cutting board. Where do I go? AliExpress? Walmart?

Local retail stores are always an option, plus you get to make your decision based on more than a grainy or staged picture and likely faked reviews.

More importantly, the customer can't tell if there is commingling.

There is very likely a person or group at Amazon that considers this a feature, at least when product commingling was introduced.

In an ideal world with no counterfeits, product commingling is an efficient thing to do, and if a product is commingled, there is no point in giving the 3rd-party seller top billing, the customer should feel like they're buying from Amazon.