So, actually this happened to me, some co-worker came up to me wanting me open the passcode of his iPhone, he claimed that he found it in his car.
It looked barely used and had no SIM.
I told him I can do it but i ll take time while I actually wanted to try to know who's it's owner.
I called Apple but that didn't help, so I offered him 300$ for it, he accepted.
I paid him 100$ and told him if I open it I ll give you the other 200$, the dude was fired from work and he never came back to ask for his money, I did everything possible to return the iPhone to it's owner but I wasn't lucky with that.
Now what should I do?
Read more: https://iphoneus.org/faq/stolen-iphone-for-100.html