Totally Turn off iPhone X without using the screen
The screen is totally black but the phone is working because the clock alarm still works, I need to totally turn off and send it back to the seller. Thanks in advance for your support
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It will eventually run out of battery and turn itself off. I don't think it makes much difference if you pack it up while it's still on.
See also here:
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support