According to Nathan Weinberg, Bill Gates checks the value of his house on property appraisal site Zillow, which values the sprawling estate iin excess of $130 million, but Gates says, "If you bid that number on my house I won't sell it to you." If Gates isn't selling, It demonstrates Zillow's value, not only as a buying or selling tool, but also as a reaffirmation of one's own value... whether it is underpriced, overpriced, or just right.