The article with options for older macOS 'pre-catalina' offers a few differences.
And also info on how to erase and re-install, in varying degrees; w/choices too.
First up, install a new system; and then leave it before it finishes so new owner
can personalize that on first-run startup. (See #7 in following for this aspect.)
• What to do before you sell, give away, or trade in your Mac - Apple Support
https://support.apple.com/en-us/HT201065
See how to erase your hard drive and reinstall macOS. (slightly less than next link)
(However if that runs into difficulty, you can use info in next link to see how macOS
Utility, in Recovery, can be useful if needed; to erase, format & re-install new system.)
• How to reinstall macOS from macOS Recovery - Apple Support
https://support.apple.com/en-us/HT204904
And very old MacBook series, may have original DVD install disc; that may be useful.
Good luck & happy trails!🌞🎣