Any idea what this Bauer compressor is worth?

I have the opportunity to buy this compressor and would like some input if possible. The owner is willing to fire it up and fill my small 90 cubic inch tank as a demonstrator. He goes on the day that it’s never seen commercial use and is willing to be available to me for technical support in the future. I’m guessing it’s overkill considering that he previously used it to fill his personal dive tanks, but I like overkill. What would this group consider to be a good price on such a unit?
https://flic.kr/p/D43DQE
https://flic.kr/p/215bH3j
 
Thanks for the info Centercut. I just spoke with the seller and got some more info. He’s got some spare parts (he’s got 8 new filter carts), at least one unique (or sounded like it may be unique) service tool/tools and manuals on it. I did get one thing wrong from our first conversation, and that is that he’d fill my tank. It’s not set up to fill through that type of valve of course. We had talked about the scuba tanks he’d just sold and I must have just misunderstood. My mind was turning the whole having my own compressor thing over, and at my age concentrating on more than one thing can be a challenge. :)
I'm going to mull it over some tonight, but considering that the asking price is significantly lower than your suggested ceiling, I probably should just buy the thing. 
 
From what I can see from the pics this looks like it might be a Utilus 10 possibly built in 1980's, so this is an older model. Nothing wrong with this fact but it does matter how many hours it has on it. They do have a rebuild schedule and duty maintenance schedule that will add to your cost. I would find out how many hours it has on it or at least how many hours since the last overhaul?
One thing I like about these models is the three cylinder "tee" configuration. None of the cylinders are facing down, therefore no water or oil will tend to "sit" on the head of that cylinder. Nice feature. They are lubricated by a throwing pin but that is expected on these smaller units, a common feature on most smaller compressors. Typical capacity of about 3.5 scfm at 330bar and the filter cartridges are designed for about 15 hours of use, depending on ambient temps.

The US Navy evaluated the U10 back in 1993 and found them to be very satisfactory for their use. You can see the report at this link if you like.
http://www.dtic.mil/dtic/tr/fulltext/u2/a274229.pdf

As far as what to pay? I would really have to know more about the hours of use and the history of maintenance. Personally if it has average use and has been maintained, I would be hard pressed to pay over 1k, just in case you have to put some bucks into it. It really boils down to the hours on the compressor. Hope this helps.

EDIT: I did look at the pics a little closer and noticed they have a scuba yoke rather than a DIN fitting on the fill whip. What this means to me is that they were filling scuba tanks to about 3000psi rather than Hi pressure DIN tanks to probably 3500+ pressures. All this indicates is that the compressor does not have to work as hard at the higher pressures in its recent history? Does not really mean much but something to consider. (hardest on compressors to run them at higher pressures for longer periods of time). Also confirmed that it appears to have been built in 1981.
 
Thanks for your input as well, FunGun. I talked to the owner today and committed to buy it. He After speaking with him a second time yesterday, he confirmed that it is a Purus, so I was able to do at least some research last night. You’re right about it’s use. The owner was a diver and used it to fill his own tanks. It fits, as he had several aluminum tanks, some older assorted scuba gear, and a fair bit of underwater photography gear for sale too.

I don’t really shoot my pcp air rifle enough to justify this thing, but I figured that if buyers remorse gets the better of me later, I can always re-sell it and buy a shoebox. I doubt that’ll happen though because I am about as big of a sucker for nice tools as I am for nice firearms. 
 
https://flic.kr/p/DaEK7U

Brought it home last night. Now to figure out what fittings I need to replace both (one not shown in photo) fill whips. The previous owner had it set up to fill to 3k on the lower pressure whip and 4K on the higher one. At first I thought I might just cap off the lower pressure one and crank the high up to 4.5, but maybe it’s be cool to have use of the lower pressure side for topping off the fun directly while I’m filling my tank/tanks. I wondered about adapting the Bauer switchover device to male high pressure quick disconnects and set up my fill whips with female quick disconnects. Both fill whips that came with the compressor have combo SCUBA/DIN fittings on them. I’m assuming that when I get a SCBA tank, I can use one of those on one whip, and one the other whip have dual female QDs for topping off my pony tank or gun. Good idea?
 
One idea, I have male Foster type QDs on all my tanks, filters, guns (with probe for FX) and compressor. And female Foster QDs on both ends of my hoses. Makes it easy to attach and fill anything, and with a male to male Foster can extend reach of hoses if necessary. Just my two cents... If you buy SCBA tanks and a fill rig, you can always fill the tank through the fill rig by using the male to male Foster adaptor.