I've linked an interesting Bally currently for sale on Ebay(I'm not buying right now, so bid-away!). It's a good case study, this is how I look at it: Not particularly rare, but an important machine as it was the second "multiplier" (allowed 1-3 coins per pull)and it was the first with multiple award lines (vs just center line pays), has the upper "topper" glass (very rare to get that in the deal), reel tapes look original (good), upper award glass has some fading on middle left red arrow (not too bad - but could be worse in person; hard to tell if not back-lit), reel glass is peeling on both sides of denomination indicator (bad, but this glass is fairly common, can probably get for $20-30), Las Vegas Hilton glass is correct for age of machine (good, but hard to prove actual Hilton pedigree - fun machine for Elvis collectors), bell is there (upper right of inside cabinet shot - good, means this probably is all original), casino card still afixed (good, rare to get that - usually there is a mark where the old card was glued), seller noncommital about operational status of machine (good - might get a great deal, bad - there's probably a number of suckers out there who might bid up the price). Finally, it's in OH, so if that's within driving distance; you'll have a big advantage over other bidders. *REMEMBER - when bidding on slots, it's crucial that you don't bid in advance; there are a lot of people out there that will take your bid as a vote of confidence & you will just be encouraging them to raise the price. Last-minute bidding is crucial to getting a slot at a good price. I recommend bidding the most you're willing to pay, with 10 seconds remaining (let the chips fall where they may)...

There are 2 books by Marshall Fey you'll want to get before buying your first Bally. For information about the different machines:

http://www.libertybellereno.com/bookpagesshort/ballyguide.html

and for repair help(upper left, in yellow, title should read "1964-1980":

http://www.libertybellereno.com/bookpagesshort/booksbally.html

These sometimes are available on Ebay, but you're unlikely to find them at a bargain price, so I'd recommend you just get them from the source, new.