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A friend of mine runs this company and I have talked to him about why they use asking prices vs. closing prices to view market trends. He says they've done a ton of research, and the two are highly correlated, so asking prices are a very good proxy for what is happening in the market - what you see happen with asking prices will be reflected in MLS statistics. And since asking prices are available on a much more timely basis than closing prices, you get a better ability to see trends earlier.
I also think his stuff is also much more accurate than what you see at housingtracker.net - since he sells it, instead of giving it away for ad revenue.