If a Person is registered in one state purchase goods from Unregistered person in other state, then he is required to take take registration in other state also to discharge GST under RCM.
So you have to take registration in Tamil nadu and then pay tax under RCM in Tamil nadu.
It seems under GST trading with an unregistered person is going to be very difficult. The whole intention of government is to bring every person carrying business under GST.
There will be 2 economies. One registered and another unregistered.