I paid about £400 I think for mine about 3 years ago and it wasn't in brilliant condition but it was complete and in good working order with a minor crack in stock and it shoots well and a lot of original finish left but I wanted to own one so I was prepared to pay. They do not often come up for sale.
It seems this one you are looking at has been tampered with; I wonder what damage has been caused by stripping - screw heads etc; also why would anyone feel the need to take it apart if it was working? You would need to spend some time and money restoring it but make him an offer of £250 or so bearing in mind it isn't working and potential damage is unknown. Does he know anything about Greeners and their potential worth?