I have purchased 14 Tesla's since 2013, getting ready to buy my 15 soon. The only advice I can give to everyone is to inspect the car before you pay in full. Tesla will push back if you ask to see it before paying and say no but then simply tell them you will not pay until you look at it first. If they dont allow you to see it then walk.
The main issues I have had with buying used X's is windshield ghosting and that stupid bottom trim pieces that are always bent or falling off. Tesla has always fixed both but it's been a battle.
I love Tesla and the Model X but depending on the service center and who you know it can be a pain in the ass to deal with them.
Once again, don't pay until you inspect the car once it arrives.