With the instructions from the linked ASC discussion applied to the computer
there would be no former owner name in the computer. The setup first-run of
the computer after a new installation is where that gets personalized; a full
replacement installation of the computer's original OS X version would be one
that it shipped with. That may be a download from the Apple Server, or if the
computer shipped with system install-restore DVDs, by use of those discs.
Could you and a trusted friend arrange to meet up with this new owner at an
Apple Store and have a genius appointment (if he'd show up with the Mac)
to have the genius re-set the OS X and have the computer in his name?
That would remove you from one aspect of the problem, as it should've been
done right the first time. A new non-personal user Admin account also works
when a new offline system is installed, with an older computer that had no
need to go online for a replacement system software. I preferred offline media.
I've set up a few computers, donated restored ones with fully functional OS
and software installed, and included the original DVD or CD media, but did
not personalize the computer. That way, it could be seen as functional and
not have my information in there. Sadly, the latest OS X isn't that easy...!
Good luck with the learning experience. 😐