ARTSY recently published an excellent article by Martin Gammon, "What Auction House Estimates Really Mean - and What They Can Tell Us."
The setting of auction estimates is often seen as a mysterious, but it really has a lot to do with understanding the psychological state of the seller and buyer, and may or may not well reflect the actual potential market value of the objects up for sale at auction.
This article is the best I have seen that explains the process of how auction estimates are set and what they really mean.