I have sold MacBook air

Additionally, contact the buyer - if it is still associated with your ID, they will not be able to use the app store, buy or update apps or erase and reinstall the OS. Here is what you should have done (and still need to do):

What to do before you sell, give away, or trade in your Mac - Apple Support

jongdo Said:

"I have sold MacBook air: I have sold MacBook Air last week, and it shows in my I cloud. can you remove the MacBook Air in my account?"

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If you sold it...

Today - Secure your Account - ASAP:

With Privacy & Security being of High-Concern here, consider performing the following for Apple and all other accounts, that have such options:

• Modify: your login credentials (opening a new eMail Account and usinge a never-before-used Password)
• Associate: a different method of payment to your account
• Enable: Two-factor authentication for Apple ID
• Take a Screenshot: of it, and send it to Apple. Include the sender's information. Froward it off to Apple at: reportphishing@apple.com

Log in to your AppleID on a browser:

https://appleid.apple.com/#!&page=signin

Check your Devices list, find the Mac, click it then click Remove from Account.