iMac 27 inch 2013
My New Computer asking for password
My New Computer asking for password
You should not be at that window if you set it up according to this Apple document: What to do before you sell, give away, or trade in your Mac - Apple Support
If prepared properly then it's first turned on it should show this screen:
I reset my old iMac to factory settings to give away, now asking for password at sign in of "My New Computer"
I may be missing something but it sounds like that should be for the new owner.
iMac 27 inch 2013