trouble preparing my MacBook Air for trade in return

• 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:

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Turn on your Mac and immediately press and hold Command (⌘) and R to start up from macOS Recovery.
3. While in macOS Recovery, use Disk Utility to repair your startup disk.
4. If Disk Utility found no errors or repaired all errors, reinstall macOS.
5. 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.