I'd just add that if you're buying a game that isn't working 100% be sure to bring someone along that knows pinball a bit, they can help you figure out what might be going on with it and whether you're taking on a $100 fix or a $500 fix. You should be able to find some local people through Pinside that would be happy to help you out.
Otherwise you're probably better off buying a game that's 100% working. Of course you'll still need to confirm this by reviewing the manual ahead of time (you can find them all on IPDB) and then playing some games with the glass off and/or working through the test menu to make sure the key functions are all there (you can find a "things to look out for" list for many games here on Pinside that will help steer you to common problems). And give the boards in the backbox a nice, thorough look (bring a flashlight) for burnt connectors and battery corrosion. If you see either, even if the game is fully working now, consider mentally budgeting a few hundred extra for board repair/replacement if the boards fail a few weeks or months after you get them home.
Also know yourself -- if you pick up a $1500 game with broken plastics, cracked ramps, damaged but functional mechs, etc., are you going to be satisfied to clean it up as best you can on the cheap and enjoy playing it without replacing every last thing? If not, you could easily end up spending hundreds on largely "cosmetic" replacement parts.