iPhone linked to Apple ID despite it's not in the device list

Hi guys,

I've got the phone, seller doesn't see it in his device list and despite of it is still linked to his user ID. What I(he) need to do to activate this Phone in my possesion?

thanks guys. Hopefully I don't have to follow silly advised from strangers to unlock this phone. It's not easy to contact the seller now as he's on different continent now and doesn't respond to my messages.

Thank you.