Buying a private used car in another Island

Anybody here has experience on buying a used car from a private person in Honolulu and shipped to another island? Looking to see what is the most efficient way for both parties to get the car dropped off to Young Brothers. Seller agrees to help with shipping.

I understand that Young Brothers requires the registration or title and insurance to be provided during the process. It would be ideal for me to just fly once and pay for the car and have the seller drop it off at the dock instead of me flying twice to pick up the car then drop it off at the dock myself.

I am not sure if I the seller has to keep the title or registration to drop it off, or do I keep the documents.