Key TakeawaysCarMax shares jumped after the used-car dealer reported quarterly earnings that topped analysts' expectations, as sales in the wholesale market rose and costs were cut.The biggest U.S. used car retailer also reinstated its stock buyback program in the latest quarter.The company said vehicle affordability challenges continued to plague the retail used car market.CarMax (KMX) shares took off after the biggest U.S. used car dealer beat profit forecasts as it sold more vehicles in the wholesale mar ...