I have my view and you have yours , if private sale prices are too cheap there is a hidden issue 99 times out of 100 . I have dealt with issues involving the miss selling of cars , stolen cars , categorised cars , fraud involving cars , cloned cars , rung cars , fake docs , errors by DVLA etc for cars for over 30 years , but then what do i know 
The point was (and still is), that you have to judge every car on its individual merits. This particular car may well be under-priced and hiding something, but the fact it's cheap doesn't ensure it's hiding something.
Not sure where your 99/100 value comes from but I must be extremely lucky, as in quite a lot of years (though, granted: I'm not as old as you
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), I've frequently bought cars under market value and never yet had a bad one. You just have to buy carefully and check them and the seller out. Sellers choose their prices for all sorts of reasons - just wanting to beat a trade-in price, needing a quick sale etc etc.
OP - fingers crossed that this one is 1/100!