I picked this up a couple of days ago for $100.
It had some issues, but for $100 I couldn't pass it up.
The compressor head made air, but wouldn't go past 50 PSI. Someone had broken the filters off the head sometime in the distant past, one head gasket was obviously blown out. It's a Worthington industrial pump, so parts are available. That's good.
It has a 5hp, 1725 motor. Starts and runs perfectly. It alone is worth more than $100. Good.
The tank is a bit of a basket case. That's bad. Decent rust pits, and overall kind of crappy looking. Bad.
Are you ready for the ugly?
Notice anything about this plumbing? There isn't a blow off valve to be seen. There also wasn't one anywhere in the tank. It's a good thing it wouldn't build pressure.
Also worth noting, there was no pressure switch to be found either. It was wired straight to the plug. 
Apparently this tank didn't have machinery mounts on it so someone added them. A finer welding job I have never seen.
I particularly liked the creative use of filler materials.
In the end, it was worth $100 just to take this thing out of circulation. It was all kind of dangerous. I have stripped the machinery and will be cutting up the tank for scrap and to make sure some other poor soul doesn't try to make a compressor out of it.
I'll be keeping my eye out for a used tank to mount the pump and motor to.
Keep in mind kids, compressors can be fairly dangerous. 120 gallons of 125psi air could probably blow the walls off my shop.
