The supercargo was an individual found on many trade ships during the age-of-sail period whose responsibility it was to represent the interests of the corporation or financiers involved in the trade goods being sold or acquired. Wikipedia says
A supercargo (from Spanish sobrecargo) is a person employed on board a
vessel by the owner of cargo carried on the ship.1 The duties of a
supercargo are defined by admiralty law and include managing the cargo
owner's trade, selling the merchandise in ports to which the vessel is
sailing,1 and buying and receiving goods to be carried on the return
voyage.
Another definition can be found here, in a list exploring roles found on ships:
The merchants’ agent on board. He could also be the freight agent, and
was responsible to take decisions about what to load and what to pay
for it.
If you search the term on our site, you will find several mentions discussing early trade (Russian-America Company, Hawai'i, California) and one specifically concerning trade with (China). These will give you an idea of some of the varying instances where supercargoes have been mentioned historically.
The first time I encountered the term was while reading Dana's Two years before the Mast', which I recommend if you want to get a feel for 19th century sailing ships, trading, and some info on early California History. Dana mentions the supercargo as a peer of the Captain, which was lacking on his ship:
The captain, in the first place, is lord paramount. He stands no
watch, comes and goes when he pleases, and is accountable to no one,
and must be obeyed in everything, without a question, even from his
chief officer. He has the power to turn his officers off duty, and
even to break them and make them do duty as sailors in the forecastle.
When there are no passengers and no supercargo, as in our vessel, he
has no companion but his own dignity, and no pleasures, unless he
differs from most of his kind, but the consciousness of possessing
supreme power, and, occasionally, the exercise of it.
Since your example was early trade with China, there is a source found here, which is by Hosea Ballou Morse
who was a customs official and historian. An excerpt from his article published in the English Historical Review, mentions the importance of the supercargo and Chinese trade:
TWO hundred years ago the supercargo was an important person on a ship
trading to the Indies. The ship went exploring to new countries; there
were no banks of exchange, and her owners had no correspondents in
foreign ports; they loaded on the ship what was required, in goods or
in money, to buy a cargo of the products of the foreign country; and,
as they could not go in person, they must have a representative on
board who was qualified to sell his 'stock', to exchange his money for
the currency of the country, and to buy his 'investment' of such
quality and at such prices that the commodities could be sold at a
profit on the ship's return to its home port. Besides this mercantile
qualification, he must be capable of dealing with persons of rank and
dignity. In one port the principal trader might be the king of the
country (such a trader was the king of Tongking1); in another he
might be the admiral commanding the naval defences, as at Mindanao in
1686,7 or the general commanding the garrison, as at Amoy in
1684;7 in another the merchants with whom he must trade were only
the commercial representatives of the highest officials, as at Canton
in 1699,7 or one merchant might have the imperial commission to
monopolize the trade with foreign ships, as at Chusan, Amoy, and
Canton in 1702–4.7 The supercargo needed diplomatic ability to deal
with all such extraordinary situations; not simply the courage to
resist extortionate demands, but the skill to conduct a trade
notwithstanding that such demands were made.
Note that not all ships had a supercargo, but if you were looking for an induvial whose only role was control of the trade on a sailing ship in the 'age-of-sail', this would be a relevant term to research.
