Reset Mac mini to factory settings

Startup in macOS Internet Recovery Mode. Option-Shift-Command-R

see > Use macOS Recovery on an Intel-based Mac - Apple Support

Connecting the Mac mini directly to your router using an Ethernet cable is easier, faster and more reliable than using Wi-Fi.

Then follow the Erase and Reinstall directions,

at > Use Disk Utility to erase an Intel-based Mac - Apple Support

mike-n-mary Said:

"Reset Mac mini to factory settings"

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Successfully Reseting a Mac to Factory Settings:

Perform as instructed here, including the backup create iCloud logout: What to do Before you Sell, Give Away, or Trade In your Mac - Apple Support