not yet happened to me , but imagine this...
you find the old ford of your dreams, hidden in a barn/garage/lock-up or tesco car-park, the elderly owner is prepared to sell, and does on of two things. either...
a, he says "i know how much these things are worth, don't you go trying to con me..." but then mentions £300 for an immaculate low mileage capri or escort. or...
b, he says "promise me you'll take good care of the old girl, i'd hate to see her all done up like a boy racers car"
in the first example, could you sleep at night, knowing you got the car for a tenth of its value? in the second, how are you going to explain what you've done to "his" car in six months time, when he sees it with its shiny new paint/cage/wheels/YB turbo!?
just a thought