I have a factory stock regulated .177 marauder LW barrel. Holds air and works as it should. In very good condition. I have manual and 6 mags to go with it. $400 pick up in NE ohio, or shipping on you.
My first thought is chrono it and see if it's speed/valve related if that's good clean barrel. Or even clean barrel first if you don't have a chrono. I'm learning these things so I'm sure there's other opinions.