There is no silver bullet for a consumer to determine a market value.
As you observed, aggregate websites such as Zillow and Trulia publish numbers that are absolute nonsense regarding the value of any specific home.
Even the public accessible tax data bases can not address this issue.
A recent check of the tax data based showed real 'sold' prices differed from the 'market value' number shown on the tax records by a factor of 0.7 to 1.8!
However, you will discover that homes typically sell within 10% of the price for which they are listed. Many are within 4%. Check with a professional in your area, Jimmy provided his telephone number, to get the real numbers you can work with.
Best of Success,
Annette Lawrence, Broker/Associate
Remax Realtec Group.
The "estimates" on various Real Estate sites are just that...Estimates. The real fair market value of a property is usually said to be determined by what a buyer is willing to pay for it on the open market (or in today's tight lending standards, what a bank will be willing to loan on it). You can also compare the estimates to the County's Tax Assessment as another tool to help determine a homes value, but overall the best way to narrow the range is to consult a REALTOR; we do this everyday. Good Luck.