• If your Mac starts up to a question mark - Apple Support
https://support.apple.com/en-us/HT204323
"A folder with a flashing question mark means that your startup disk is
no longer available or doesn't contain a working Mac operating system."
~ If the question mark persists, follow these steps:
- Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
- Turn on your Mac and immediately press and hold Command (⌘) and R to start up from macOS Recovery.
- While in macOS Recovery, use Disk Utility to repair your startup disk.
- If Disk Utility found no errors or repaired all errors, reinstall macOS.
- If you still need help, please contact Apple Support.
Without knowing exactly where this went off the track,
you may have to consider Erase and reinstall macOS.
• How to reinstall macOS - Apple Support
https://support.apple.com/en-us/HT204904
Certain steps and button choices to re-install the OS
can install different versions of supported systems, too.
So to try again, and consider whatever resulted in the
outcome experienced; and try to plot different course.
• What to do before you sell, give away, or trade in your Mac - Apple Support
https://support.apple.com/en-us/HT201065
And read toward end of instructions, see Step #7. This
allows your Mac to have new system: Not personalized.
"After installation is complete, your Mac might restart to a setup assistant.
If you're selling, trading in, or giving away your Mac, press Command-Q to
quit the assistant without completing setup.
~ Then click Shut Down. When new owner starts up the
Mac, they can use.. own information to complete setup."
(.Not sure how that may help a 'non-buyer'. This doesn't
help to get you past the immediate situation, just now..
Those answer(s) are among varied instruction details.