I too bouhgt my first Form 1+ a little over a year ago, used from a private party, and it has worked well, until recently.
Just this past Saturday, I bought another one, and it’s working great.
As far as what to look for, the display will not show you much. If you press and hold the power button for a few seconds, the display will show you the firmware revision and the serial name/number, but other than that it’s not very useful.
If you can ask the person to open the unit, you could pull out the SD card, and look for the odometer file. That could give you quite a bit of information, like the laser time, galvo time, peel motor time, etc. That would give you an idea of how many total hours of printing it Form 1+ has.
But even that, will not tell you if the printer is in good working condition, or properly calibrated. Only a sample print can tell you that.
Remove the resin tray, and look inside. If you see any resin stains, that means resin had found it’s way inside (leaks, spills), and that means it had some issues.
AS the seller if the printer is still under warranty. If it is, you can always send it in to FormLabs for repairs if there are problems. If it’s out of warranty, any repairs will cost you $800, regardless of what’s wrong.
Last but not least, the asking price will determine if it’s a good deal or not.
I paid $1000 for the used Form 1+ I bought the other day, and while it was not under warranty, I was able to see a fresh print coming out of it, and the quality was good, so it was worth it to me.