Things to consider before buying used iPhone?

I'm going to meet up with this person whose selling his white 4s for $350. I saw his ad online (Kijiji) and he just got it 2 months ago, so that means that there's 10 months of warranty left. He's selling it because he upgraded to a 5. How can I know if the iPhone is in good condition, physically and internally? How would I know if it had water damage or not, or any kind of bad things that he did to it? I don't want to get scammed so what are the things I should consider before buying a used iPhone?

Added (1). Also, how would I know if its a stolen phone or not? It comes with the box and everything but it isn't sealed

Check the conditions of the phone in the description. The box isn't sealed because he already opened it.

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