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You would need the charger for sure. I traded in my Mom's series 2 last year and had to bring the charger and make sure it's wiped and not linked to anyone's Apple ID or iCloud accounts. I would bring the box if you have it just to make it easier for them to find the serial number since they will have to put it into the computer to find the trade value for it.
 
When I traded in my iPad they didn't want anything but the device. I walked out with the charge brick, cord and box. Not sure about the watch so you should bring the charger just incase
 
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