many of my pens are second and even third hand... never had any issues with the nibs. the reason I buy used pens? Because they're often cheaper. Because I don't believe most of the claims that we alter the pens as we use them - certainly not in the short term. I'm at odds with "experts" but given my experiences, i see no evidence for this change at all. I've never even, to my awareness, noticed a difference in how I perceive a *new* pen from the first time I write with it, to the 50th time.
Perhaps I adapt quickly to how a pen feels and make minute adjustments of my own that compensate for any minor changes another user has wrought. But i'm highly skeptical of that. I'm just not that sensitive.

There is *one* exception in my experience. A vintage pen some 50 years old did have a slightly flattened spot on the left tine of the nib tipping. I'm guessing that it probably had one owner for a great length of time who happened to have the same grip and rotation I have ("average" grip and rotation for right-handers probably) and that sweet spot was perfect for me. But this was an inexpensive school pen (perhaps softer tipping than what is used today) that had been used for decades. I've never seen any such thing on any other used pen I own (some of the same age as that particular pen.)
since this ties in heavily to the thread that folks disagree with so much on - "don't ever loan your pen" and an earlier thread on the same topic, I'm sure this answer has the potential to go down that road yet again. But it's the only answer I have for you from a technical POV.
From an emotional POV, using a pen that has been in someone else's arsenal for many years carries with it a history that many find intriguing. For me, that's of minor consequence but many users/collectors put more stock in it. Check out Angelo's very thoughtful thread on this topic ... uh..somewhere. I'll have to do some searching to remember where it is.
found it:
http://www.fountainpennetwork.com/forum/in...?showtopic=9189and to reiterate - a used M200 that might have minor wear is perfectly acceptable to me for $10-20 cheaper than brand new as I am a user, not a "collector" who needs MIB-condition pens.