There are other barometers I'd use (at least for an E500E, or a 560SEC, which are cars that I know/follow) -- the Hagerty valuation, SCM valuation guide, NADA, Kelly Blue Book (if applicable), and other sale sites.
I think BaT is "easy" because of current/recent sales, searchability, and transparency of "sold for" prices. So lazy people just go there, but they do not tend to objectively evaluate what their car is actually worth based on condition, history, deferred maintenance needed, and so forth.  It's very easy to just say "well my car is that same year, color and looks to be in the same condition as that 500E that sold for $XXK on BaT, so mine must be worth that too."
Even on BaT there is significant variability.  I mean, a 
@deanlaumbach 500E is going to bring an automatic $10K premium due to cosmetic gussying-up (mechanicals and deferred maintenance be damned) over the same exact car on BaT offered by a generic seller.  And that same car (as with the recent Dean 500E) would be available on eBay "Buy it Now" for 50% less just a couple or three months earlier.  So say a car is a $50K car from an average BaT seller, brings $60K via Dean, but is a $40K BIN on eBay.  50% is a wide swing in price for literally the same car.
Guess it makes the case for all sellers here who want to maximize the price if selling their car, to use Dean to get a 25% lift !!