You sold an m-series Mac.
Thus, the new owner can use the "erase all content and settings" option, which will do what the new owner needs. It will "wipe it clean" of ALL of your data, remove it from your iCloud, etc.
HOWEVER, there's a catch.
In order to run this option, the new owner is going to need YOUR administrative password.
Are you willing to provide it?
This feature will not run until a password is entered.