The machines are close to you right? if so just go over there and check them out and
play a few games.

Are you talking about gameplay or condition? I guess either way the answer is still play the game and run it though the diagnostics. Personally it was my goal to find a game with a few problems, that way I'd be able to find a better game still in my budget. I would have loved to come across that TZ you posted, at the right price. If you don't have the confidence in knowing something is wrong when you see it or fixing it for that matter--I might suggest buying from someone that a user here suggests. That way you can get a nice fully shopped machine.

You'll eventually have things go wrong, but maybe you can even talk with the guy that you are buying it from about what type of problems he's had in the past and how he fixed it. I've owned my machine for only one month and I'm constantly tinkering with things. "That ball didn't trigger that switch, why not" (investigate) "That light isn't working, hopefully it's just the bulb" (investigate) etc etc etc...

Sorry to all the women on Pinside (please substitue all the he/guy/etc. for she/woman/etc. as needed.)