One thing you want to pay attention to is condition ,
Playfield and backglass are #1 & 2 but its also whats inside that you may want to avoid.
if you can take a look inside the machine look for anything that appears odd
such as lamp wire, more then one transformer, parts with other mfg name on it.
well you get the idea you dont want to bring home someones 'hack job' even though
your new don't be hesitant to ask questions if the seller starts getting defensive maybe he has something to hide?

just remember - "Never slow down, never grow old"
when it comes to pinball