I own a variety of "used" guns. The vast majority of those were bought used because they are no longer in production. I would not call them "collectibles" just yet, but they were guns that I wanted that could only be found on the used market.
Of the used guns I have purchased, I have only run into problems with two: a S&W Model 28 that was out of time, and a WWII issue Colt 1911 that had been butchered by a shade-tree gunsmith. (Bob Hunter had his hands full making that thing run at any semblance of normal. Seemed ok to me on first look, but the previous owner - or somebody before him - had taken some metal off the feed ramp...)
Moral of the story is, in my experience most used guns have been fine. The two that were not would have been caught, if I had used Jim March's Revolver Checkout on the Model 28, and if I had taken time to check out the Colt more thoroughly before firing it.
So, buy used, but know what to look for; where applicable, do not fill out the 4473 until you have checked the gun out.