I received refurbished AirPods and the original owner still has them connected to his Apple ID, how to I fix that ?

You don't. The original owner must disconnect them from his account.

Activation Lock is designed to prevent anyone else from using an Apple device if it's ever lost or stolen.

Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner can remove the lock by using their Apple ID & password. Apple will not help you find the previous owner. If the previous owner cannot or will not remove the lock, then the device is useless to you or anyone else. Try to get a refund (through eBay) or dispose of the device properly.

If you know the previous owner, here is what he must do:

• With a web browser, go to: https://www.iCloud.com and sign in with his Apple ID. Open Find My iPhone and select All Devices (at the top). Then click his AirPod, select 'Erase', and select Next until the device is erased. Finally click "X" next to remove his AirPods.

Then you should be able to use them.

Try this.

How to reset your AirPods and AirPods Pro - Apple Support