Disclaimer: I am not the voice of experience.
You could just outright approach them on it, if it's not working it's not making them money, and as far as a buisness is concerned that means that some space that could be making money somehow is presently a loss. Just ask to speak to a manager, inquire if they might have interest in selling it, and if so whom could you contact? Even if they weren't thinking about it, I'm sure they'll at least entertain the offer.
Thing is though, it could be sitting because they're waiting for a part, or it could be a vendor that put it there and splits the money. If they're waiting for a part then they're not unhappy with it, it'll make it a little harder to get. If it's a vendor...the vendor's out to make money, and he knows the value of the machine pretty well most likely, you probably won't get a deal.
Looks like on E-bay they're going for between 1k-3k working cockpits, cabinet looks about $750. I'd lowball it, first ask if they know what's wrong with it, and make a guesstimate on how much it'd cost to fix. Adjust your offer by there. If it were something minor, I'd say "Well how about $400(Or maybe $500)?" and see what response I get.
If it's a hesitant "Ummm...I think it's worth a bit more...", they're considering the offer, you'll get it for like $100 - $150 more probably.
If it's a straight out "No.", it's going to be a bit more, ask them if they have a number in mind that they're willing to part with it for, then offer like $100 - $150 less and negotiate. Any number they give you is going to be one to two hundred more than they'll actually accept. Just like you will lowball them, they'll highball you.
I have a couple collector-type hobbies, I'm used to negotaiting, it's not hard, it's really alot like poker...