You let a mate park a car on the path for a week, spend all bloody day fixing it so it runs, freeing off the brakes, sorting the overdrive, and generally fettling it electrically for him to turn round and say "Well, I bought it for £350, cost me £40 to get it home, but now it's running I want a grand for it" when you offer him £500?.
Tempted to go round and bill the bastard £500 for the work that I've done to it.