What calibre?
What woodwork?
What condition?
What pressure is the bottle?
They all make a difference to sale-ability and price. A while back, I sold a .177 HarrierX (not X2) in as-new condition, with an ambi beech stock for £300 - obviously that was gun-only, but that seemed like the going price.
Depending on the answers to the questions above, I'd say somewhere nearer the £450 mark myself.......
Just my opinion.
Dunk
"Every normal man must be tempted at times, to spit on his hands, raise the black flag & start slitting throats"