Can I trade in an iPhone if is not mine?

As far as Apple is concerned the phones are owned by the person who bought them and locked them to their Apple ID.

No legitimate company will accept activation locked devices for trade in.

Trade in values are based on the ability to resell the device.

Locked devices have no resale value.

AFAIK, Apple uses a third-party for evaluating trade-ins. As part of the process, the phone must not be activation locked so the evaluators can actually check out the phone. I would guess that these "lost" iPhones would not be accessible, and as such, would not provide you with any trade-in value. In addition, they are not "your phones" to trade in.