The National Association of Realtors said on Monday existing home sales increased 4.9 percent to an annual rate of 4.89 million units. Still, the increase in sales will be welcomed by the Federal Reserve, which is closely watching the housing market as it contemplates the future course of monetary policy. Fed Chair Janet Yellen has warned that a prolonged slump could undermine the economy. First-time buyers, a necessary ingredient for a strong housing market, continue to hug the sidelines.