Just make sure everything works as it should. Ask the seller what bills it takes and then test it. Hopefully the machine has a DBV200 (or a WBA) and that it, at least, takes $1's and $20's. Does it have a hopper? If so, check to see it works by playing enough for a pay out. If there's a ticket printer, and no hopper, press cash out to make sure the ticket prints and the machine doesn't go into a hand pay mode for something like a 10 coin pay out. You can pull out the MPU and check the battery but it the machine works, there is no need to in my opinion. If it goes bad later, they're easy to replace. That particular battery usually doesn't leak like the Varta battery in the IGT S2000.
Check the power cord. They get really frayed. If it's just cracked a little, with NO wire exposed, you can wrap electrical tape around it. Open and close the door and make sure it's tidy inside and that there's no obvious broken bulb sockets.
If you get the machine, make sure the hopper is removed. In many cases, people put the machine flat in a truck or van and any stray coins fall out and can wedge behind things like the mother board! This can cause bad stuff to happen.

When you move the machine, insure the doors (main and the belly glass) are CLOSED. It's easy to grab the machine and accidentally unlatch the door. Sometimes the belly glass door isn't latched all the way. If it's not, it can fall open and break the glass.
Tell the movers to be careful.