I have bought a MacBook Air from our yard sale, which doesn

“I have bought a MacBook Air from our yard sale, which doesn: I’m trying to lock a MacBook Air that I bought from a yard sale which doesn’t have the password or pass key how can I get through that?”

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Changing the Password of Another’s Mac:

You’ll have to ask the person currently signed into their Apple Account on it to sign out on their own.

Learn from This:

This is the site the seller should have used, prior to selling it: What to do Before you Sell, Give Away, Trade in, or Recycle your Mac - Apple Support. They should have known better.