Bought second hand phone but can’t find original owner to turn off Apple account off purchased phone.

Return it and get your money back.

If you can’t do that, and you did not buy through a service that offers some form of buyer protection (e.g. eBay), toss it in the nearest electronics recycle bin and take this as a life lesson.

There is no way to bypass the activation lock.

Return the phone for a refund.

It is completely useless without the original owner's Apple ID and password.