Cleaning MacBook Air early 2014 for sale

Reinstall and leave it at the configuration step so the new owner can continue from that.

> Have tried them all and don't get any further.

In what way did that seem to fail?

On Sept 6, I tried to directly download macOS Big Sur from How to download and install macOS - Apple Support

and I got a message from the Mac App Store indicating that version of macOS is not available. I did this on a 2020 iMac.

That early 2014 Air shipped with macOS Mavericks which Apple does not provide for reinstallation. My same year Air has macOS Catalina on it and with 4GB RAM, it is a dawg. Probably should not go beyond Mojave on that configuration.

What you do before you wipe a Mac is determine what operating system can be downloaded from the above link, build a bootable 16 GB USB stick, and then you have a reinstallation option when all else fails.

Sell it in its present condition. Anyone interested in a decade+ year old Mac will know what to do.