I mentioned this in another thread, but I bought a used Craftsman oilless compressor that turned out to be a decent buy, but the crack head looking couple that sold out lied about the condition of it. It was a Craigslist ad with a couple of crappy pics. The seller (wife) replied to email saying it worked, etc.. come check it out.
I showed up at this house at night in a bad part of town and 'husband wasn't home yet'. The wife came to the door dressed in a nightie looking big t-shirt, and seemed skeevy - the kind of gal that looks pretty good for a mug shot. After she secured the giant dog in the house, I nervously went into the back yard and saw it in a rickety shed with no lights. I scanned around for meth lab stuff, but didn't see any. She awkwardly tries to shine a flashlight in the shed. The guy gets home, meets us in the backyard, shows me the compressor, etc..it was very clean, but he 'didn't need it.' While he was pulling it out of the shed, into the light, down a home made wooden plywood ramp, the guy slipped real quick right onto his back, like out of a cartoon. He laid there like he was dead for a second, then just groaned about his back. Talk about awkward.... He got up, I gave him the cash and hurriedly loaded it up and left..
Got it home, ran it the next day and found that there was no compression. Damn.. we had turned it on, but hearing it run, I figured all was well.
I took it apart - the cylinder/sleeve was badly scored, the compression ring/piston was completely trashed. Had to replace the cylinder/sleeve, compression ring, related o-rings. Seems like about $30/parts from Sears Parts Direct. Also added a nice valve on the bottom from Lowes. The petcocks are usually garbage, even on the more expensive ones.
Point is, buy used for a great price, beware with oil-less that you may have to repair it.