Remove old user

See these articles What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Trade in with Apple GiveBack - Apple

You have the details of the seller and you can only contact the person to get Apple ID address and password .

No one here can help you and Apple can't help you.

You need the ID and password from the last user or the device is useless to you.