First-time buyer advice

I'm buying my first gun, and as I'm strapped for cash my buddy offered me his California compliant M&P Shield for $325 bucks.He tells me that his "airforce buddy who is good with guns made the trigger less mushy" but he doesn't remember exactly what he did to it. He doesn't feel comfortable contacting the guy to ask for specifics due to a strained personal relationship.It sounds like some polishing occurred, but what should I look for when I disassemble the gun before buying it? As long as it goes bang when I want it to, and doesn't go bang when I don't, I'm happy. It will be my primary self defense piece.Should I spend more money and just buy a new gun? via /r/CCW http://ift.tt/2DixYf2