I've crossed the 400,XXX mark a few times on my older cameras. All 1d series — 1d MKIIn had 430,xxx before it died.
I have never had any that have died outright, just started showing the brighter horizontal line down the middle of the frame.
I do find selling a used camera much like selling a car, everyone is very curious about the shutter count. Although the mileage of a car is very well documented on the dashboard, you most likely need a third party app to tell you the shutter.