Sold me an locked iPhone 7plus in Miami, how do I find the owner?

Find My iPhone Activation Lock - "…—a feature that's designed to prevent anyone else from using your iPhone, iPad, iPod touch, or Apple Watch if it's ever lost or stolen. Activation Lock is enabled automatically when you turn on Find My iPhone."

http://support.apple.com/HT201365

Turn off Find My iPhone Activation Lock (formerly: Activation Lock: Removing a device from a previous owner’s account) - An activation lock can only be removed using the log-in information for the account that applied the lock.

https://support.apple.com/HT201441

Get the owner of the account to remove the lock or return the device for a refund. The device is otherwise useless to you.

If the owner has not provided contact information as a message showing in the login screen, there is no way to contact the owner unless the person who gave you the device knows them.

No way to do so if you cant reach that "someone".

Bring it to the police station.