I'm with pesto on this. Unless you know the person and know how the gun performs and have seen it in person, it's often not worth your time or money. Some of the price people ask are outrageous and not far from new price.
Just buy it new so you don't have to worry about stripped/overtightened screws or poorly installed shit.
buy new gun.. shoot it till it breaks, then decide if you want to fix the gearbox or put in a new one, generally fixing is better if you know a good tech. Drop ins are never guaranteed to fit perfectly with a 'close enough' industry standard for manufacturing tolerances.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.