Refuses to go into wifi recovery mode

Try connecting the laptop directly to your router using an Ethernet cable and a USB-C to Ethernet Adapter.

If you have access to another Mac you can create a bootable macOS USB installer:

https://support.apple.com/en-us/HT201372

If the WiFi icon is missing it would seem to indicate a hardware issue so you will likely need to have Apple or an Apple Authorized Service Provider examine the laptop and provide an estimate for repair.

Do you have access to another Mac?

If yes, log in to the Mac App Store with your ID and download the macOS. If that other Mac supports Big Sur it is better once your Mac does too and you can use the newest version. If not, It must be Catalina then, the oldest version supported by your Mac.

Once downloaded, create a bootable installer How to create a bootable installer for macOS - Apple Support

Restart your Mac holding Option and choose the installer.

It might be you need to change the security for external boot media About Startup Security Utility - Apple Support. If you need to do that, then do it and reboot with Option to load the installer.

Once up, open Disk Utility and erase the disk How to erase your Intel-based Mac - Apple Support

Close DU and proceed installation.

You can also install the macOS on an external drive on that other Mac and then use it to boot your Mac and give it a fresh start.

After macOS is installed on your Mac, just don't set it up. Leave it that way so the buyer can continue the setup with his own information.