If your are purchasing a new system and sending the Mac mini for trade-in, you should get detailed instructions from Apple on what to do with the Mac. They will also send you a shipping box to return it—after which, if it passes inspection, you will get the credit for the Mac. Primary points you will need to do is first, turn on Find My Mac, then you need to erase the drive. I don’t recall if I had to reinstall macOS or not.
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