[NoPiece]

I think that is a little misleading. They may intermingle, but only in the sense that products share a product page. But you always explicitly see who the seller is, and if you buy from Amazon (seller), you will never get a product from a third party.

The problem is that if you don't know to pay attention to the seller, you can end up buying from a third party while looking at a first party product page.

[JohnTHaller]

It's not misleading. Look up Amazon product comingling for more details. A given product with a unique UPC has the stock from all sellers as well as Amazon itself combined by default with no distinction between who contributed which.

[NoPiece]

So, even when you buy from Amazon itself, you may get one of the goods shipped to the Amazon warehouse by a third party dealing in counterfeit goods.

That is a misleading statement because Amazon only commingles third party sellers, not their own inventory. If you buy from "Amazon itself" you aren't at risk of getting commingled inventory.

[JohnTHaller]

According to the Wall Street Journal's reporting on it in 2014, orders from Amazon itself were also affected:

"The system has enabled Amazon to make better use of its warehouse space and keep a wide variety of items in stock around the country. The idea is to give Amazon flexibility to ship certain products based on their proximity to customers, speeding delivery times. For third-party sellers, it saves them the trouble of having to label individual items sent to the Amazon warehouse. The bar code takes care of that. But the practice has in some cases led to mix-ups between counterfeit and authentic products, even when they are sent by Amazon itself."

That would appear to be why Wusthof knives, for instance, forbids any authorized retailer from commingling and doesn't count Amazon as an authorized retailer.