Editor's note: This report is based on Realtor.com's August 2012 Real Estate Trend Data Report. The report covers 146 U.S. metros, and includes single-family homes, condos, townhomes and co-ops. The number of homes for sale nationwide fell 18.68 percent from a year ago in August, to 1.84 million, Realtor.com reported today, continuing a trend that's manifested itself every month so far this year.Year-over-year trends in median list price and median age of inventory, particularly in California, also indicated many housing markets are beginning to stabilize.The median age of inventory was down in August (11.65 percent) from a year ago to 91 days, while the national median list price was up a slight 0.5 percent from last August to $190,000. For-sale inventories declined on a year-over-year ...