I've bought 3 used commercial grinders and most recently a La Pavoni Zip.
Here is how I assessed it when I looked at it with the experience of the other 2 I bought.
Before you go look at it price out any replacement parts it might need. (hopper, lids, burrs, doser parts, etc). Go with those prices in your mind.
Remove the upper burr holder and look at the burrs. (the La Pavoni burrs were shot). Subtract $50 for new burrs in your head.
Outside condition, the La Pavoni was beat, repainted then beat again. It was filthy. Everything was there (hopper, lids, doser, switch, etc.).
Run the motor. How noisy is it, does it start quickly (does it hesitate). Turn it on and off a few times. Feel and smell the motor afterwords. A shot motor will get hot and smell. The La Pavoni motor seemed good.
Test the doser (surprisingly the doser parts can cost alot). The La Pavoni doser was complete, but sluggish, and filthy.
Once you have an idea of what it will cost you to fix (time and money), see if you think it would be worth what you'd pay for the grinder and the repairs.
The La Pavoni was priced at $275. I offered them $200 cash and explained I though it needed a couple hours work, new paint and new burrs just to be usable.
All-in-all I spent 3 hours on it and $50 for new burrs and $7 on a can of good spray paint (I used Rustolium hammered paint). I was going to have it powder coated but that would have cost another $75.... not worth it to me.
I fully disassembled the grinder and sandblasted the frame, cleaned everything. The result is it's like a new grinder now.
The biggest effort was disassembling, cleaning and re-assembling the doser, but it's spanking clean and working properly now.
$257..... new these are $850 - $1500