First off, I would get the serial number of the back and drop Phase One a message through their support website and check if they’ve ever had it reported as stolen (they’re helpful in this regard). Of course if there’s a paper trail back to it being purchased new (e.g. you’re dealing with the original owner) that helps, but you can never be too sure.
Other than that just the usual checks with regards to used gear - what's the chain of ownership - any dings or dents - does the wear reflect how it was used (amateur vs. professional) - any warranty left - do all the buttons work - any dead spots on the touch screen - take one completely black (underexposed) and one white (overexposed) frame (if you can, or get the seller to do this and send you RAWs) and check for hot/dead pixels or obvious sensor tiling, which may mean the back has to be re-calibrated - has the back ever been repaired - check for paint loss or signs of wear on the screws - any sign of battery corrosion in the battery compartment - any signs of mineral residue (e.g. salt spray) anywhere - any scratches on the sensor cover glass - any signs of paint touch up - does the general wear and tear of the back match the shot count (e.g. you would expect a back with 100,000 activations to show a degree of wear and tear, but not one with a shot count of 100) - any sound of something loose when you tip it back and forth - is it ex-rental - was it regularly serviced - do the FireWire and USB ports work ok - no loose connections - is the Firmware up to date - when mounted to the camera is there any 'play' - how was it bought originally - was its purchase financed - if so is this paid off - was it bought off lease - etc. etc.
These things are pretty much bomb proof (kudos to Phase One) and if there are no obvious technical issues you’ll almost certainly be ok - and you’ll be saving a ton of cash (since the going rate for a private sale IQ180 is around the ~$15k mark with some warranty, less without any warranty remaining).
Finally, if it’s in your nature - remember to haggle - it’s a buyers market for this gear - and with full-frame CMOS probably not that far from appearing, it's going to stay that way (i.e. the used price is only going one way, and it's not up).
Good luck with the purchase.
Jim
