With the used-car market sizzling and Carmax wannabes forming every week, Toyota Motor Corp. is the latest to repackage their up-to-four-year-old cars with less than 55,000 miles and sell them through a certified used-car program. Patterned after a similar effort its Lexus luxury division launched last summer, Toyota is extending the powertrain warranty to six years or 1 00,000 miles from the original date of sale. "I don't consider Carmax and the others a threat," says Yale Gieszl, executive vice president of Toyota Motor Sales U.S.A. "They're not doing anything a well-managed Toyota or Lexus dealer can't do."