Return it to it's original owner. It's worthless to have because of Activation Lock which requires an Apple ID password to activate. No one will ever be able to use that phone unless they know the Apple ID password or if the original owner deletes it from Find My iPhone. Sometimes, you need to do the right thing. The original owner most likely has photos, videos, and other important files that they don't want to lose. I was a victim of iPhone theft and it was NOT fun. I lost irreplaceable photos and videos.
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http://www.iphonehacks.com/2014/06/find-lost-iphone.html