Manhattan apartment sales jumped 19 percent in the third quarter from a year earlier, reaching the second-highest level since 2008, as the city’s job market stabilized and mortgage rates fell to record lows. The number of completed sales climbed to 2,661 from 2,230 a year earlier, New York appraiser Miller Samuel Inc. and broker Prudential Douglas Elliman Real Estate said today. The median price of co-ops and condos that changed hands increased 7.5 percent to $914,000 as buyers sought larger units. The market is back to a “normal comfort level,” said Jonathan Miller, president of Miller Samuel. “We’re comparing against last year when we weren’t fully off the floor. It’s currently stable.”