Question: what to do before giving away a mac?
Answer: Create a backup. Make sure that you have a current backup of your important files. ... 
Sign out of iTunes in macOS Mojave or earlier. ... 
Sign out of iCloud. ... 
Sign out of iMessage. ... 
Reset NVRAM. ... 
Optional: Unpair Bluetooth devices that you're keeping. ... 
Erase your hard drive and reinstall macOS.