Yes, for most items sold on Amazon, Amazon is just a sales forum for third party sellers ("Amazon Marketplace").

But for a large number of popular items sold on Amazon, the item is sold by Amazon itself ("Ships from and sold by Amazon.com.")

The problem is that even for items sold by Amazon itself, Amazon is not doing enough quality assurance on what it receives from its suppliers, to make sure the product is real.

For example:

Apple: Nearly 90% Of 'Genuine' iPhone Chargers On Amazon Are Counterfeit ("And since those products were sold directly “by Amazon.com” using Apple’s product marketing images, it made the products seem even more real.")

If Best Buy, Target, or Walmart were selling fake products in their stores, they could try to blame their suppliers, but they could only get away with it a few times. After a while, people would assume that products sold in store were falsely labeled and stop shopping there. Also, the store would face consumer class-action lawsuits for false product labeling.