ZILCH. Listen to what the others said. It can give you a general idea in an area with a lot of activity but thats about it. My primary real estate income comes from valuing houses for banks. The MLS is the most accurate tool we have but sometimes we will look at Trulia, Redfin, Zillow and other sites if we need to find one extra property possibly sold by owner, etc. My favorite is this site. Not just because Im in here now---Im also in Zillow and say that I like them the least.The market is changing rapidly. Its been a crazy, wild ride for the last 3 years. Valuing property now can be frustrating and in some cases almost impossible. Values are all over the board with some areas going up and others still going down. Then we have certain TYPES of property that may go up or down when their general market area is opposite.
Are you dealing directly with the seller or is this just a general question with example? I do have to say that many sellers now are over priced. We say 'Price not supported by sold data'. Some are not properly counseled by their real estate brokers. Others are trying to find an ignorant buyer, forgetting that the house has to appraise for a loan. Some have little or no equity and hope they can squeeze every last penny out. I travel 7 days a weeki around the Portland area, doing hundreds and hundreds of properties. My LEAST favorite zip codes are 97223 and 97035. Severe lack of listings and solds make it very hard to value homes. Have three brokers give you a market analysis on the same property. See how much they vary and if any of the brokers ADJUSTED for things like 2 vs 2.5 baths, square foot difference and by dollar amount per foot, age, bedrrom count, how many car garage, etc. If it comes up less than the sellers asking price, include your CMA with the offer unless you plan to lowball less than the CMA. Or, have your agent call the listing agent and ask how they came up with the price and compare their data.