From RealEstateJournal.com:

After getting five estimates — four from online sites, and one from a professional appraiser who inspected the home in person. We were told the home might sell for anywhere from $291,000 to $375,000 — an $84,000, or 29%, spread.

The appraiser is probably closest to the mark, of course. Completely automated systems like Zillow.com can be accurate by accident, much as a gift-buyer might be accurate buying you clothes using measurements obtained from your friends. In many cases, I can tell practically to the dollar what a home will sell for. Other homes defy prediction. Certainty is never hard to find, though. If you want to know what your house is worth, sell it. The home’s value is what you got for it. Everything else is just a guess, no matter how well educated.