If it's a reputable seller which is known to honor the warranty they promise, there isn't much to worry about.
nhanzero What do I have to look for the unseen defect you mentioned of?
Just check that every button works, bring a laptop or phone (and appropriate cables!!!) to test MIDI with a virtual instrument (even if you don't plan to, it's good to check, if you don't have anything pianoteq is a good for a test, or fluidsynthMIDI on an Android phone -- test both at home first so you don't waste precious time while onsite). @CyberGene can tell you the details since he's more experience with those things than I am and he has an identical unit, I think.
Make sure that you try EVERY SINGLE KEY at EVERY SINGLE playing volume (pppp to ffff and anything in between), try also note repetition. If any of these don't work perfectly, they can likely be repaired, but it's an additional expense that you may want to negotiate, and a small risk that it might not be repaired, so be prepared to negotiate with seller if you discover any (obviously a problem in the highest or perhaps lowest octave would be very different than in the middle of the instrument).
Yes, do try with or without headphones and just see if you enjoy the tone and touch of the instrument.
It may be useful to google the instruction manual and study it beforehand, so you know your way around (I believe some functions are a bit convoluted, or at least so they were on the NU1 -- I actually like as it was, but it require you to know and learn them before)
Best of luck!