I bought my iPhone 5 second hand on Gumtree. It was in perfect condition and the seller showed me a receipt and we had a lot of communication before hand when we met up. I did thoroughly quiz him about every aspect of the device first...focusing on the cosmetic and origin of the device...ie. was it an upgrade? Pay N Go? From Apple Store? Unfortunately though it developed a speaker fault (buzzing noise) and had to be exchanged at a Genius Bar for another, but this was a week after I bought it. (The Apple replacement had scratches but that's a whole different story)
I would recommend getting as much information as possible about the phone. Where it was purchased, proof of some kind, ID of the person, plus either a full description of the condition or photos or definitive answers to your questions. (eg. Are there any scratches/scuffs on the metal part of the phone, including minute imperfections?) If they give you short answers or ignore them then you can tell a lot about the seller. If you meet up to purchase it, meet in a public place (Starbucks or such like) and if you inspect it and find there are marks on it, don't be afraid to refuse it. Make this clear when communicating to them prior to meeting up. I also insisted the seller un-register it from their Apple Support Profile page so you can re-register it to yourself. This will make it easy if you need to go to Apple for another issue, should one arise.
All this is based on if you physically meet up, if you plan to get it delivered via post I can't comment on that, only to say I would never buy it without seeing it first, so it's not an option for me!
