Sorry, i have no idea of value but the lack of power could be the main seal being long since dead..
It be that the seal is dry as a bone and need a bit of oil introducing, also, there's probably a pellet pusher that screws in at the rear, these used to have a fibrous sealing ring so that the air diedn't escape around the threads..
It's similar to one of those Dollar type pistols or the modern day gat guns i presume...
It looks in quite nice condition
Cheers, John