When it comes to second hand shoes, I would normally advise against ever buying something that doesn't still have the tags on. I think it only takes a couple of wears for a shoe or boot to begin molding to the shape of one's foot, so the chances are a second hand shoe isn't going to fit your foot comfortably.
Having said that, one of the girls I work with in the charity shop loves buying vintage shoes, and she has never had even a tweak of pain while wearing a pair. Perhaps it's nothing to do with comfort, just the thought of one pair of sweaty feet in the same place as my sweaty feet (gross).
I made one exception to my rule a few weeks ago, and bought beautiful Kurt Geiger soft-leather knee-high boots from my charity shop. These were £20, and after a long inspection, I decided these could only have been worn once, so sensing an unbelievable bargain, I splurged, scooped them up and carried my almost-new babies home:
Why did I make the exception? Well, first of all, Kurt Geiger. Second, I'm pretty sure many of the shoes we buy from shoe shops have been bought before, tried at home then returned. Is there really much difference with these? There wasn't a flake of dust on the sole, my only reason for thinking they weren't completely new was the inner label was a little squashed and deformed. Bargain in my books.
So, would you buy second hand shoes or boots? More importantly, do you? Or do you need that new-shoe smell to feel satisfied that your pieds have found their perfect match?