Is this Arduino MEGA 2560 R3 a counterfeit?

NotForth:
but when they ship they will send anything other “sellers” have listed as Arduino."

No, that is not true. Amazon will only sell from the seller you select. However, the non-authorized sellers are linked from the CanaKit page. This is the same for all Amazon products. So it wouldn't take much to order from the "wrong" seller.

If you click "Add to Cart" on the page that is stated "Sold by CanaKit" then product will be fulfilled from CanaKit's stock. If the buyer selects another vendor from "more buying choices" then that's the inventory that will be used to fulfill the order (assuming Amazon is still doing the fulfillment.)

So yes, Amazon needs to be addressed along with the actual seller. Amazon does need to take responsibility for the sellers that are misrepresenting products and correct/remove those listings.