saw this and thought of - VTiredeyes
negative equity.
what happens if, say, someone bought a car at £20K
6 months later trade in value is £15K
but they still owe £16500 on finance.

can they ring the finance company up and say... look i cant afford it anymore. i cant sell it for less than trade in.

can i just hand the car back to you and forget about it?

just wondering....
saw this and thought of - Canon Fodder

Only if you GAP insurance I think VTE, and then only with a valid reason like you lost your job or similar.

Otherwise they'll be at you for the missing £1500 and would quite happily go to the County Court.

CF
saw this and thought of - Pugugly {P}
And a negative effect on an owner's Credit Rating
saw this and thought of - VTiredeyes
mmmmmmm
cheers chaps
saw this and thought of - GrahamF1
Of course this is just my rather unhelpful personal opinion, but I'd have said they were crackers to borrow that much money at car finance rates unless they were sure they could meet the obligation.
saw this and thought of - GreatestDancer
this works if you've paid off 50% or more of the initial capital.

e.g. once you get to having paid off £10k of the £20k then even if the car's worth only £1k you just hand back the keys with a smile on your face and you're able to walk away - all square.

this assumes you're talking about a hire purchase arrangement and the car's been serviced on the intervals and has no obvious damage (torn seats, dents, bad scratches)