I don't think the specific issue here has anything to do with commingling. Commingling means putting all inventory of the same item from different vendors in the same basket, and taking items at random from that basket at the time of fulfillment, without a clear way to know the originating vendor of the fulfilled item.

Here, the problem isn't that items are put together in the same basket, it's that they are counterfeit: no seller should have any "new" inventory of the book, at all; new inventory should only come from the publisher. As the publisher isn't an Amazon seller, Amazon itself needs to buy the book from the publisher, and carry it in their own inventory.

Not commingling, ie carrying a separate inventory for each vendor, doesn't change the problem at all: all inventory not coming directly from the publisher is counterfeit.

In theory, it could be possible for other vendors to buy from the publisher and then sell on Amazon; in practice this is highly unlikely. But Amazon doesn't care and doesn't do any basic check.

They are probably hiding behind the fact that the sellers are doing the counterfeiting, not them. IANAL, but since they obviously benefit from it, it should be possible to sue, not just in civil court, but in criminal court as well.