i need help ..i buy ipad mini from someone..after i do hard reset then the ipad mini need the first user apple id..but i cant find the seller until now..any solution how can i activated it?

You can't.

It's called activation lock an anti-theft feature.

If the owner of the iPad does not remove it from their account the iPad is useless.

Return it for a refund and only buy devices from legitimate sources.

There is nothing any of us here or Apple can do to help you. You have bought a useless device.