Actually, from the point of view of a Boston Merchant, the Louisiana Purchase was terrible. It gave the indication that the center of commerce of the nation would shift from the Eastern Seaboard to New Orleans; in fact the whole purpose of the purchase was to obtain New Orleans, Jefferson's original purpose. Jefferson was so concerned with maintaining New Orleans and the Mississippi River for American commerce that he once told the French Ambassador to the U.S.
the day that France takes possession of New Orleans, we must marry ourselves to the British fleet and nation.
One must remember that this was before the advent of the railroad or steam boat; all commerce necessarily flowed downstream. The Louisiana Purchase ensured that New Orleans would grow in importance for commerce, and New England would subside. This would be horrible news to a Boston Merchant. Additionally, Boston Merchants tended to be Federalists, whereas the Louisiana territory would be primarily occupied by farmers and settlers who would be Republican. Since these areas would someday become states under the terms of the Northwest Ordinances, it spelled the death knell of Federalist control.
It was, in fact, the Louisiana Purchase which led to the infamous Essex Junto, a plan devised by Boston merchants and lawyers to have New England secede from the U.S. since it's power base would soon be lost. The exposure of the Junto ultimately led to the duel in which Aaron Burr killed Alexander Hamilton.
The loss of political influence and control of the nation's commercial traffic meant that the Louisiana Purchase was to Boston Merchants the equivalent of doomsday.
For the most part, the Louisiana Purchase did not do much good for a merchant from Boston. The reason for this was that it did not contribute to such a person's business. Instead, the Purchase did more good for merchants in New Orleans. The goods from the area that was obtained from France generally moved down the Mississippi to New Orleans. They did not make it to Boston and therefore did not enrich the merchant in your question. Of course, there was not much harm done to that merchant either since the Purchase did not cost much money. In addition, the merchant would have benefited in a small way from the general rise in prominence and importance that the US experienced as a result of the Purchase.
I read a book about John Jacob Astor, which might show how a Boston merchant might benefit from the Louisiana Purchase.
The Astor family had a musical instruments business, and John came to the United States near the end of the Revolutionary War to sell musical instruments. He did not live in Boston; initially he lived in New York City.
He switched from musical instruments to fur trading. The Louisiana Purchase made fur trading even more profitable. Astor had agents in Oregon and in Minnesota, who bought furs.
I bet you might find examples of Boston merchants who profited from the Louisiana Purchase in the same way as Astor.