Carvana (CVNA) is on a roll in the stock market. The used car e-retailer, known for its towering car vending machines and fully online sales model, is benefiting from favourable used auto market dynamics and improving unit economics thanks to cost containment measures.After a whopping 280% surge last year, the stock is up 50% so far this year. Just over the past month, shares of Carvana were up 26%, outperforming close peers like Sonic Automotive (SAH) and Lithia Motors (LAD) . Shares of Sonic Automotive an ...