They think there is more money in parts than the whole unit.
The same thing I ran into when buying my Mach1 as it was advertised as a parts car not complete.
When I went to look it over it was minus the eng/trans and rad only.
The fellow had made a list of all the parts of it and (to him) what the parts would sell for each.
With his figures the total possiable would be about 12k where as the best offer he had gotten for the whole car was only 4k.
Now after haggling and alot of back and forth and finally convincing him how long it may take to get to 12k for the parts, We settled on 6k for the complete car.