Well, first of all you want to look at the reason the person is selling the gun. Upgrading to something faster or leaving the sport are good reasons--"I fuggin hate this piece of sh*t!" means DON'T BUY IT! Ask people that know the guy selling it if his gun has worked well for him. He may tell you anything you want to hear, but if other people around him say "Yeah, he's kicked ass with that gun. Never down. Shoots straight." do consider it. Similarly if you hear "That POS went down on him last month, cost us the tourney"; "yeah, poor spent 2 hours fixing it last week", etc. GO FIND SOMETHING ELSE.
Also, test it for yourself in a couple of games to inspect it yourself and see if it's right for you. The times that I haven't done that I have been royally screwed over. Everyone selling you something will say "Yeah, I love this thing, it rocks." But if it doesn't fit well and peform for YOU, on YOUR FIELD, IN YOUR HANDS, then say no thanks and walk away.