The National Association of Realtors said Monday that sales of existing homes fell 6.1 percent to a seasonally adjusted annual rate of 4.93 million. The combination of higher home prices and relatively stagnant incomes has reduced affordability and restrained buying activity. Lower affordability, resulting from tight credit, rising home prices and essentially flat incomes, held back sales for the rest of the year. Analysts say sales of roughly 5.5 million existing homes are common in a healthy real estate market. A sudden blast of cold weather and snowstorms at the end of November might have cut into sales, noted Ian Shepherdson, chief economist at Pantheon Macroeconomic. Rates have fallen as investors are plowing their money into 10-year U.S.