A coworker is selling his iphone and i think of buying it for my sis because she broke her last one?

Now, what should i ask from him to know that this is not a stolen phone? Any papers or what? I have no idea when it comes to apple products

You should test it out yourself. Maybe go meet him with your sister and the three of you can discuss something out. Your sister must know an awful lot since she used it before. Or if she can't go in any case then tell another friend to go with you