It is better to be sure that you check if your iPhone for stuff like that especially, if you buy it from somewhere besides a technology store or depot. If you get it from Apple or Best Buy it shouldn't be anything close to an issue or an risk to you.
If you are not buying your phone from an authorized dealer I believe you should check to if the phone is stolen. I would feel really bad if someone stole my phone. I would not want to contribute to the loss of another persons property. Always purchase your phone from and authorized dealer if you don't want to check.
A stolen iPhone could have any number of problems with it in addition to it simply being someone else's rightful property. For one, here could still be private information from the original owner on it. It could also have software on it designed to capture your personal information for the seller's use. Plus, a person knowingly selling a stolen phone isn't likely to be truthful about any existing problems or damage to the phone.
I would never recommend that anyone buy black market goods because there are too many risks associated with doing so. For example, a stolen phone could potentially be linked to a crime committed using that phone, there could be debt connected to that particular handset, and of course the most obvious thing is that it's morally wrong and you are essentially supporting crime if you buy a handset that is stolen.
Black market iPhones have to be cut out from reselling and reusing once the initial owner reports the theft. Some sort of tracing and locking software might help limiting iPhone Black Market sales. It` s a double negative situation: Black market existence encourages thefts and affects negatively the producer as the resold devices go out for lower prices.
Its mine and its the sellers fault for stealing it. If my phone was stolen yea i would be mad but if mine wasn't stolen why should I care, if other people got it stolen it's their fault for not being aware. But Verizon and At&t wouldn't steal so i need not worry.