When an auto loan is in default in the state of NC is the buyer or co-buyer primarily responsible?

Both the buyer and co-buyer are equally and completely responsible. All parties on a contract are 100% responsible to pay the note in full per the terms of the contract. Neither person is "more" responsible than the other. My suggestion is to help the bank find the auto and they may cut you some slack on the any remaining balance after the sale out of good faith but they have no obligation to do so.