Its a toy rather than an airgun , I think ( ? ) but as I have just taken an interest in pop outs cannot get sniffy .. although pop outs are mainly airguns .
I think this is an elastic band powered toy gun ? Hence the preloaded 'top slides' .
Mid 50's at a guess then later .
As with Gats , made ad infinitum so exact dating difficult .
As for value am sure somebody will want one - from £20-£ £30 to whatever they are prerpared to spend , suggestion £60-£70