A home is worth whatever a buyer is willing to pay and a seller is willing to accept. An experienced, busy real estate broker, knows best how to determine the true value of a property because they are working with buyers and sellers in the current real estate market, and they have access to the best data (from the MLS). They will also correctly adjust for quality and feature differences.
The second best valuation source is an experienced, busy appraiser. They have access to the same data, but they work for banks, not buyers and sellers. In addition, they are constrained by inflexible appraisal rules which do not allow them to consider some relevant information and comparable properties.
Inexperienced real estate brokers and appraisers can be wildly inaccurate. Web sites like Zillow.com calculate market values without the input from human experience or judgment and they use incomplete data from county tax records. The “value” found in the county tax records is not useful, because it is calculated for the purposes of property tax assessment. Further, in the Raleigh area, this number is updated only once every eight years, the last time being January 2008. Up until that time most properties sold for far above tax value. Since then I have seen properties sell for $100,000 above and below tax value.