It's a housing market that's being measured not in sales, but in the rate of decline. Finally there's some good news for sellers -- a floor is forming for some of the most volatile markets.

According to the Altos 10-City Composite Price Index, real-time asking prices for homes declined just 0.3 percent in May. Some may look at that and say, yes, prices are still falling, but look at the momentum. Prices of properties listed for-sale increased in 15 of 26 major markets according to the Real-Time Housing Market Report and Real IQ.

Prices are still volatile in vulnerable markets like Las Vegas, down 3.7 percent during May, but prices rose by the same amount in Denver.

Overall, prices increased by more than one percent in 11 of 26 markets. This is no easy feat considering that listing inventories rose in 25 out of 26 markets, or 3.7 percent in the 10-City composite. The surprising exception was Miami, where inventory actually declined 1.3 percent in May.

"The weakest markets continue to be concentrated in Florida, California, Arizona and Nevada which all experienced the biggest price increases during the boom times," said Michael Simonsen, CEO and co-founder of Altos Research. "Now these same markets are outpacing others on the downside as foreclosures and price declines continue relentlessly. We're seeing some stability in many markets outside of these hard-hit states."

For the Altos 10-City Composite, the average days-on-market was 106 -- an improvement from 111 in April and 113 in March. Miami experienced the longest days-on market at 152, while San Francisco had the fastest rate of inventory turnover at 73 days on market.

One reason why the Real-Time Housing report and 10-city indexes are more positive than other indexes such as the Case-Shiller is that the information is not based on sold data or same sales data, but tracks real-time listing prices. Also included is a wider range of cities, some of which are less volatile, like Austin, Houston, and Indianapolis.

Data in the Real-Time Housing Market Report is based on current analysis of over one million properties currently listed for-sale in 26 metropolitan markets across the country. The 10-City Index is composed of 31 major MSAs.