They will have usually a few cocktails, and operators are less likely to bid on them unless they have a buyer scheduled back home. Likewise, they will have older stuff mixed in with the newer stuff that operators are less likely to bid on since they are interested in the money-makers. However, there will be plenty of sentimental collectors out there too. So chances are they will have some cocktails, but chances are they won't be cheap.
So your best bet for a machine would be an older less popular game that neither an operator nor collector will want really badly. I've heard lots of stand-up driving games fit in this category. You can also find deals on early 90's games since they aren't considered classics, but they aren't big cash cows anymore. Also, slightly older fighters will sell for cheaper since the newer fighters will make more money on a route and collectors generally aren't interested. These often make the best conversions because of larger monitors and expansive control panels (except I personally think Campcom machines are ugly).
I'm pretty sure that sometimes they will auction off non-working machines, but I don't think they accept incomplete machines. So if you do buy a machine be prepared to pay for everything in it - including the parts you don't need.
Yeah, I would like a cocktail cabinet, but I think these are one category overrun by collectors. Still, it doesn't hurt to look...