Hi. I like these, and have one, which I keep for nostalgia. Your G10 is probably not firing pellets properly because it is old and tired inside. These were never that powerful in the first place however, as were basically a BB plinker for children, and even when brand new, seldom mustered more than 0.75ft.lb of power. So to value. Well imo, in mint condition and boxed, maybe £25 plus post absolute tops? But as is malfunctioning as described, a fiver posted max. Hope this helps . Atb: G.