I just bought a reburbished Mac but it has someone else’s Apple ID on it

You can't.

You have two choices: either you return it to the seller and ask to then use the password to sign out of iCloud so that you can sign in, or you you have to find a space for a large paperweight.

See How to remove Activation Lock

Briyikes Said:

"I just bought a reburbished Mac but it has someone else’s Apple ID on it: How do I sign in to a Mac that I bought on back market when it has someone else’s username/password on it?"

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Return This:

Have this person logout of it. If that is not an option, then return it. The seller should have known better.