I think if I was going to sell OC'd PC's, I would be sure to use a very good cooler, and I would be a little conservative with the frequencies. Probably set the speed more like 3.5GHz. I'm running my own E8500 at 3.8GHz, no problems with it so far, and low temperatures with its Zalman 9700. I need it to run fast, but only for limited periods of time so it doesn't sit there and keep generating heat. But someone that hits the CPU hard for long periods of time would obviously be putting more strain on it.
Another thing I just thought of- what if they use the machine in an environment with high ambient temperatures? This would affect the ability of the cooler to keep the CPU cool.