Thats where the "haggle like a northerner" comes in.
Your arguement is
I won't be able to sell it as well so I am not paying asking.
Can we run some engine tests please
I figure it will need a new engine soon so need to reserve cash for that
It will drink oil and water
Its more expensive to insure.
Once all these are done they are either going to take an offer to move it on or clam up and back away. If they sell it cheap then you need to push harder as they are desperate and there is a reason for that. If they clam up then they think its worth more and so hopefully have some morals.