Factory resetting a MacBook before selling it

The step people skip is signing out of iCloud first. If Find My is still on, the buyer hits activation lock and can't use it, and you'll be getting messages. So: back it up, sign out of iCloud in System Settings, sign out of Messages, and unpair any Bluetooth keyboard or mouse you're keeping. Then, on an Apple silicon Mac or a newer Intel one running Monterey or later, System Settings > General > Transfer or Reset > Erase All Content and Settings does the whole job and leaves it ready for the next owner. On older ones, boot into Recovery, erase the internal drive in Disk Utility and reinstall macOS from there.