I just bought my car using Car Max and had a great experience. The first car we bought actually turned out to be a lemon, which could have been a really horrible experience, but Car Max took really good care fo us through the exchange process and we just picked up our shiny red Jeep Renegade last night and are totally pumped. I really recommend them over dealerships especially.
Carvana is great! I know so many people who have bought from them and had great experiences. While I haven't personally, my mother & sister both have. They love the service and their cars were perfect!