Sales of existing homes fell last month to the lowest marks in four years, according to a report released today by the National Association of Realtors.

The pace of purchases fell 3 percent in March to an annual rate of 6 million homes, the first time sales fell below 6 million in three years. Sales are down 11 percent since April 2006.

The drop in sales added to the inventory of unsold homes on the market. Currently, it would take 8-and-a-half months to sell the nation’s existing unsold homes for at the April sales pace. That’s the largest glut of unsold homes in 15 years.

Sluggish sales and an increasing inventory means home prices are down. The association said the median price of an existing home sold in March was $220,900, down just under 1 percent compared to March 2006. That’s the ninth straight monthly drop for the price tags on existing homes when compared to a year earlier.