First let's assume that Amazon can't sell new cars due to restrictive franchise laws in every state.
You are easily off by an order of magnitude on the billion dollar business statement. Carmax is the largest used car dealer in the nation, with a market cap of over 11 billion. As of 2015 they captured about 1.7% of used car sales. (They sold 582,282 vehicles to consumers and over 33.8 million used vehicles were sold, I'm on my phone so I can source this later if you'd like)
Second Amazon is an beast and anyone in a market that Amazon is considering should pay careful attention. That said I think that vehicle sales is such a different market in many ways that Amazon will either take a long time to figure out the whole process and provide a consistent quality product or they will acquire a company currently in the used vehicle market. The two leaders in the online space had a huge leg up due to ancestry or acquisition. I doubt Amazon would be stubborn enough to learn the business the hard way.