bought an open box iphone

They might be able to do this for you at an Apple Store. Or, you can ask Best Buy to contact the owner (maybe).

You'll probably have to ask Best Buy to contact the prior purchaser and have them unregister it. To do this, the would do the following:

1. Go to: https://supportprofile.apple.com/MySupportProfile.do and login using their Apple ID.
2. Click on ">" to the right of the phone listing
3. Click "unregister"

I would call Apple Care and explain the issue and ask them to release the phone from the previous Apple ID. They will likely ask that you email or fax a copy of the Best Buy receipt to verify that you are now the owner of the phone.