How do I unlock activation lock when the previous owner is out of contact?

here is a support article.. you'll find the sections that indicate you must have the help of the previous owner

How to remove Activation Lock - Apple Support

Sadly you will never be able to use it. You can recycle it at any Apple Store. Read this link...

The All Too Common SAD Reality of Buying … - Apple Community

You can't.

you won't be able to use it.. you return it to whoever you bought it from..

You can't, you have been scammed by the seller. Either find them and get your money back or report it to the police.