Gah, wait until you get a rep for figuring out poltergeist infested guns. Then airsofters bring their stuff to you after they've already poked at it and had a doctor or two give up on it leaving behind their own little surprises. Spring and bushing upgrade is pretty straightforward. Try chasing down the symptoms of compounded wear issues in an auto GBB or an AEG built of aftermarket parts from 3 makers and you can blow a few hours on a worn down ASG that's worth not much more than your time at $25/hr.
Then add in time for waiting in line at the P.O. to ship a gun back as well as the logistics issues of getting parts.
Want nearly free GBB gas?