I don't know anything about the US housing buy/sell market, but I have some little experience in my country (Italy). Appraisal of houses is extremely difficult

I don't know anything about the US housing buy/sell market, but I have some little experience in my country (Italy). Appraisal of houses is extremely difficult because there are both objective data (like size, quality of construction) and many very subjective (at least here while you can draw some rough lines distinguishing neighborhood/areas in a city, sometimes a house just around the corner has much more - or much less - value) and there are what I personally call "financial capability of the masses" (in a given area/city). Say that you have a house that is appraised by several people to have a value around 400,000 Euro (or dollars, anyway "units"). What a young couple (both working) can afford is 350,000 Euro max, let's say 100,000 Euro cash and 250,000 loan. So you have out of 100 people - willing to buy at the moment - 98 of these young couples and only 2 other people (these latter that can afford the 400,000 Euro or more). Your house suddenly is not worth anymore 400,000 Euro because most of the houses (similar to yours and valued anything between 360,000 and 420,000) after - say - six months that are unsold will lower the asking price, and your asked 400,000 will appear way too much and you will be forced to lower it. The other way round, if your house is valued 300,000, there are good chances that you can sell it for 330/340/350,000, because there are 98 people out of 100 looking for exactly that price range in the area, and as soon as one or a few ones are sold in the neighborhood for 340,000, all similar houses will go from 300 to 350,000. And this tends to be cyclic, at a certain point that particular area/neighborhood becomes (for whatever reasons) trendy and prices skyrocket, then a couple years later (again for whatever reasons) it goes down to the bottom. Everything is very ephemeral and fluctuating, maybe what Zillow attempted to do (and seemingly failed) was more to become a sort of monopolistic entity capable to "impose" prices?