Amazon is selling ground beef for $1.99 a pound while the national average sits at $6.89. They lose money on every single pound, and it's one of the oldest plays in retail. Grocers have known for decades that shoppers only track the price of a handful of items. Milk. Eggs. Bananas. Ground beef. The industry calls them known value items. Price those impossibly low and customers assume the whole store is cheap, then fill the cart with 40 other products where nobody remembers what anything…