Your are right on the money. The short answer to your Q is they cannot set an accurate value and they can hurt sellers badly. These sites are for the buyers.
All the best,
Drew from Zillow here. While Zillow does help connect buyers with sellers with an array of social features such as Zillow Discussions and Home Q&A, I want to make it clear that Zillow does not sell leads.
For anyone who wants to learn more about Zestimates or see Zestimate accuracy statistics on a county by county basis, please click the url below.
Zillow and Trulia are not realtors and are not appraisers. They are computer based applications that use a very complicated mathimatical formula to analyze data and come up with a value. Rarely are they correct but yet people seem to continue to use them. Any buyer worth having will know better. Be patient and the right buyer will find your house.