These days, I strongly advise customers to personally view the machine. If you are not sure what you should be looking for, bring someone who does.
For this game, check the DMD and make sure it has no missing lines. If you are familiar with the game, play the game for sale and make sure everything is working. Check the boards - any hacks ?
Look for simple stuff like broken ramps, plastics and wear around the flippers and outholes etc..
Make sure it comes with the correct size and type of legs.