The price of existing homes last month fell 2.2 percent, the largest monthly decline in the almost four decades the number has been tracked, according to an industry report released yesterday. Nationwide, the number of existing single-family homes sold fell 14.2 percent in September compared with September 2005, according to the report from the National Association of Realtors. The number of sales has fallen each month since March. Prices fell everywhere in the country, with the Northeast and West most affected. Declines were more moderate in the South, which includes the Washington area....