Interestingly enough, in the abscence of a purser or quartermaster, the captain was often responsible for these duties.
Captain William Bligh was the Bounty's purser, due to the lack of any other officers on board. He was responsible for selling additional goods and services during the voyage. He did so without any profit to himself. Bligh didn't want to cause any warm feelings or irritation by exacting high profits for the sale of certain extra items. He stated that his commision was enough.