Bought refurbished iPhone 6

You can try to restore it through iTunes sure.

But you may run into Activation Lock if BestBuy did not remove any Apple Id logged into icloud during their refurbishment process.

Which if they left an active passcode on it, was likely not a very thorough process.

If you do run into activation lock, you will have to return it toi BestBuy, as there's nothing to do then.

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

Don't be surprised after this if you run into Activation Lock.

Where did you purchase it?

Personally I would return it and suggest fraud on the part of the seller to see if that gets your money back.

If not, it is completely worthless to you.

Refurbished by whom?

Was the iPhone set to the Hello screen, and asked to be setup?

Or was it already at the Home screen (the one with the Apps)?

What exactly happened?

Hi,

You can’t without contacting the previous owner so they can sign out of their Apple ID.

Turn off Find My iPhone Activation Lock - Apple Support