It is VERY RISKY buying a used iPhone. You don't know if it is locked to a carrier. If it is you don't know which carrier. You don't know if it has been hacked to unlock it - If it has, then it can never be upgraded without relocking it. You don't know if it has been reported stolen. There was a post yesterday from someone who paid £500 for a used iPhone, then it asked him for his Apple ID when he tried to activate it. There is a feature in iOS 7 that will only allow the owner (as determined by Apple ID) to activate the phone, to make iPhones less desirable to steal. Apparently this phone had been upgraded to iOS 7 (possibly illegally), was then lost or stolen. So now it is permanently worthless to anyone, and the buyer is out £500.
Some savvy buyers ask the seller for the serial number so they can check its status. Then, when they get the phone, find that it has a different serial number than the one they were given when they asked. There are multiple posts in this forum on a daily basis who have bought used iPhone and found that they can't use them.