I bought a used macboook on eBay. It has a firmware lock. How can I use the MacBook?

You cannot. Return it in accordance with eBay's policies.

For reference: Set a firmware password on your Mac - Apple Support

duke50bhc Said:

"I bought a used macboook on eBay. It has a firmware lock. How can I use the MacBook?: How to use a used MacBook with a firmware lock."

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Getting Info a Locked Mac:

You have been scammed. This is the site the owner should have used prior to selling it: What to do Before you Sell, Give Away, or Trade In your Mac - Apple Support. The seller should have know better. So, get on their case by contacting eBay.

Without the password from the original owner, it cannot be used at all. Apple takes security seriously and if there were a way to bypass the lock, it would not be secure.