Shipping. The US is huge and many parts are sparsely populated. You can free deliver in 12 hours in New York and make a profit. But you can't do that in North Dakota.
Amazon has the infrastructure to deliver anywhere in the US in a reasonable time and their size allows them to offset that guy in the Oregon woods ordering a pair of socks with a Manhattanite hipster ordering a $1500 coffeemaker.
Amazon has the infrastructure to deliver anywhere in the US in a reasonable time and their size allows them to offset that guy in the Oregon woods ordering a pair of socks with a Manhattanite hipster ordering a $1500 coffeemaker.