Bought a Cat D myself thinking I would just put it on track but am really enjoying it. Damage was virtually all cosmetic apart from a scratch on the frame where the brake pedal bent into it and a small ding on the swing arm from the exhaust.
Have seen a couple of bikes in the past that the owner has quite clearly had an accident on but not claimed on insurance. Suppose to do own repairs cheaper and avoid bike insurance history. Also to possibly keep his no claims - so they're not all squeaky clean even if history checks out and shows no claims on it.
I suppose the main thing is to check any bike thoroughly. But that can be difficult when buying on eBay.