Reset used Apple Pencil in preparation for sale?
Apple Pencil is a Bluetooth device and is not tied to the iPad its paired with any more so than Bluetooth headphones are.

Per Apple:


  After you pair your Apple Pencil, it will stay paired until you restart your iPad, turn on airplane mode, or pair with another iPad Pro.


You can also manually make your iPad forget the Bluetooth association (go to Settings > Bluetooth > tap the "i" button across from Apple Pencil > tap Forget) but that doesn't affect the Pencil in any way other than you'd need to re-pair to use it again.