Housing inventory around the country is on the rise, as sellers look to capitalize on the spring home buying season. Home sale inventory was up 2.97% in March and up 6.83% over the three months ended in March, according to the Altos Research 10-City Composite Index. The index is a statistical compilation of property prices highly correlated with the S&P/Case-Shiller Index. The 10-city index is based on single-family homes in Boston, Chicago, New York, Los Angeles, San Diego, San Francisco, Miami, Las Vegas, Washing D.C., and Denver; however, Altos also reports figures from other cities around the country. Sale inventory in Boston increased the most during the month, up nearly 16% to 14,297 properties, followed by New York (up 8.24%), Philadelphia (up 7.68%) and San Francisco (up 6.71%). Prices, on the other hand, are trending downward, Altos said. The national average sale price fell 0.29% in March 2011 compared to one year prior, down to $432,307. Over the three months ended in March, prices fell 2.3%. Prices decreased the most in Atlanta, down 1.13% to $182,276, followed by Las Vegas (down 1.03%), San Diego (down 1.03%) and Detroit (down 0.79%). Realcomp reported Monday morning that sales in Detroit dropped 9.6% in March. Write to Christine Ricciardi. Follow her on Twitter @HWnewbieCR.