I'd say either take it to the police, or take it to the carrier that it's activated on. Maybe they can find out info on the device and contact the owner to pick it up.
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It's not your place to just up and sell it online. Clearly the owner wants the device back, hence the message on the screen.
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The question there is: IS IT ILLEGAL, OR JUST NOT COOL TO SELL IT FOR PARTS?
I've already recommended that he give it to the police, but I know he's basically wondering that ^.
It's not your place to just up and sell it online. Clearly the owner wants the device back, hence the message on the screen.