Sam,

First question is are you including the camera? As for price I wouldn't be interested in paying too much as I can buy a new reversing system for around £100. Whilst it would be great to have the same unit, there is no manual for it also whilst it is very rare it is also obsolete therefore if it packs up after a week I'm back in the same boat.
I'll offer £50 for the head unit and £30 for the camera.

Col