This is the flaw here. The seller, real estate agent, banker, and appraiser all have incentives to raise the price of the home. The only person in the transaction who doesn't is the buyer. I've been working on something in this area as a side project, and have been working with the fallout of 2008 in my day job, since 2008... and it continues to this day and the lack of understanding of the incentives here is the main issue. If Zillow wanted to gain customers through their zestimates they would raise the estimate not lower it. After all, more people own homes then are buying one in any given moment. Telling those people how smart they are that they timed the market is a much better way to retain users.