Sign out of iTunes in macOS Mojave or earlier

If you’re using macOS Mojave or earlier, open iTunes. From the menu bar at the top of the screen or iTunes window, choose Account > Authorizations > Deauthorize This Computer. Then enter your Apple ID and password and click Deauthorize.

Sign out of iCloud

If you’re using macOS Catalina or later, choose Apple menu  > System Preferences, then click Apple ID. Select Overview in the sidebar, then click Sign Out.

If you’re using macOS Mojave or earlier, choose Apple menu  > System Preferences, click iCloud, then click Sign Out. You will be asked whether to keep a copy of your iCloud data on this Mac. You can click Keep a Copy, because you’re erasing your Mac later. Your iCloud data remains in iCloud and on any other devices that are signed in to iCloud with your Apple ID.

Sign out of iCloud

If you’re using macOS Catalina or later, choose Apple menu  > System Preferences, then click Apple ID. Select Overview in the sidebar, then click Sign Out. If you’re using macOS Mojave or earlier, choose Apple menu  > System Preferences, click iCloud, then click Sign Out.

You will be asked whether to keep a copy of your iCloud data on this Mac. You can click Keep a Copy, because you’re erasing your Mac later. Your iCloud data remains in iCloud and on any other devices that are signed in to iCloud with your Apple ID.

Sign out of iMessage

If you’re using OS X Mountain Lion or later, open the Messages app, then choose Messages > Preferences from the menu bar. Click iMessage, then click Sign Out.

Reset NVRAM

Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R. Release the keys after about 20 seconds. This clears user settings from memory and restores certain security features that might have been altered.

Erase your hard drive and reinstall macOS

The best way to restore your Mac to factory settings is to erase your hard drive and reinstall macOS. After macOS installation is complete, the Mac restarts to a setup assistant that asks you to choose a country or region. To leave the Mac in an out-of-box state, don’t continue setup. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, the setup assistant guides them through the setup process.