You also need a valid case. The fact that your local Ford dealership doesn't sell Chevys isn't a basis for a lawsuit.
Indeed. However, if you were the dominant reseller of cars in the country of all brands, e.g. with a 60% market share - and then started selling your own brand cars and stopping sales of some of your competition, you'd be in trouble.
Amazon would be in big trouble if they were a dominant player in the market (and I'm sure lawyers would have a long fight to determine which market...). They aren't, thus no case. Just remember that Amazon sells their own stuff rather than better alternatives, so don't go to Amazon if you don't know exactly what you're getting. Never do market research there, they'd prefer to sell their own junk rather than superior alternatives.