I've just seen two shuffle bowlers locally that I passed on, that might help for a reference. One was on craigslist for a bit, and was a 79' Taurus Shuffle Alley by United/Williams. I believe they wanted $400 or around there. Condition was fair, and I don't think the pins came down; so it would need some work. Pass on that one.
Second, was a Deluxe 6 Player Shuffle Alley by United (51'). My friends dad found it, they called me to see if I wanted it. Apparently it was in great shape, somewhat mint, didn't work but the guy wanted $300. I called back, said I'll take it; but by the time they were able to talk to him and make the deal; the guy went a bought a new fuse which fixed the machine. The price jump by a factor of 10.....to $3000. Passed on that, which sucked, because $300 would have been a great deal.
So, as Gozer says, you could just ask "what do you want for it", let him throw something out. If he say he has no idea, or make an offer, I'd go $150 - $200, as that seems to be fair. That would probably be my max, if the only guarantee he can give you that it works was, "it worked when I stored it/took it apart."
Also, good reference for bowlers/alleys:
http://www.marvin3m.com/bowl/index.htm