factory reset iPhone X
How to do a factory reset on a iPhone X (purchased from another person)
iPhone XS
Question marked as
This is what the original owner should have done...
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support
Factory Reset here...
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
If you encounter Activation Lock, you will not be able to use the device without the cooperation of original owner.