 Shares of CarMax Inc. KMX, 6.49 tumbled 9.7 toward a more than two-year low in premarket trading Thursday, after the used vehicle retailer reported fiscal second-quarter profit that fell well below expectations, as affordability challenges led to sales falling sharply in the final months of the quarter. Net income for the quarter to Aug. 31 fell to 125.9 million, or 79 cents a share, from 285.3 million, or 1.72 a share, in the year-ago period. 