Current device is registered with old owners Apple ID, will not let me sign in with my own Apple ID?

The previous owner needs to unlock it with their password, or give you the Password. Otherwise, the phone is useless to you and nobody can help you. Get your money back, if you can.

What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

Yes, only the original owner's Apple ID & password are accepted.

Give it back for a refund. The iPhone is useless to you.