Do NOT buy it.
All iphones are controlled by whoever set it up, and it has your icloud ID on it. If its stolen then no one can setup the iphone with thier own ID because it will require a password and you will NOT get around this. Dont confuse icloud lock with phone carrier lock, icloud lock is very secure.
The problem is if the seller can not remove the icloud lock then its most likely stolen otherwise he would remove this. Dont believe any excuse they give you because either way you will be stuck. You cannot wipe the phone or do anything with it and they can also see where the phone is on the map because you cant turn off find my iphone either. Its a great security feature that iphones have.
If you were selling your iphone, you would wipe the phone (and put in your password when asked) to remove the icloud lock off it. Dont ever buy it unless thats removed.