Cars are like Houses/Antiques/whatever.
They are ultimately worth whatever somebody is prepared to pay for it, there are no other rules!
If you think it is worth the money, Buy it! If you dont, dont! If there is middle ground haggle!
But if I were you, I wouldn't buy a Car from a neighbour (definitely not from a friend) as a matter of principle.
Serious can of worms potential here!
(Exception! I say to owners of older cars who are facing big repairs, It is worth what it would cost you to replace it!, Not what you could have sold it for. YES it is worth spending £700 doing a head gasket on an old Peugeot as long as the rest of it is in reasonable condition and you are happy to keep it for another 2-3 years)