Lost iPhone

Unless to owner has included a message saying how to contact the owner, you cannot contact the owner. The owner has placed the device in lost mode with activation lock which makes the device unusable to you. See if you can get a refund from the person who sold it to you.

Use the Find My app to locate a missing device or item - https://support.apple.com/HT210515

"What happens if you activate Mark As Lost

Mark as Lost remotely locks your missing iPhone, iPad, iPod touch, or Mac with a passcode, keeping your information secure even if your device goes missing. You can also display a custom message with your phone number on the Lock screen, and keep track of your device's location.

Once Mark as Lost is activated, Apple Pay is disabled, if you have it set up on the device. A confirmation email is sent to the primary email address associated with your Apple ID. When you recover your device, you will need to unlock it with your passcode."

rajat104 Said:

"Lost iPhone: I am from Nepal. A guy sold me an iPhone 11. Now it says it has been reported stolen. What do I do?"

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Hand this Over:

Be honest about it, and hand it over to local authorities.