I took it pretty far on mine. Took off the backglass checked the boards and batteries, pulled up the playfield, checked the boards there, made certain the DMD was working completely by playing 3 games and making all the shots on the board, getting the different multiballs, starting the video mode. Of course took a very close look at the playfield and bulbs/LEDs.
If a seller gets nervous about these things, I would start to question their reason for being nervous, they're about to have a lot of money in their hands. I'm just making sure the investment is sound.
Of course if the machine is already known to be a junker, I would consider it anyway, they just wouldn't get the high dollars for it.