Luchesi does not appear in the story but has an important role just the same. Fortunato does not need to go with Montresor to the underground vaults or catacombs in order to sample the Amontillado (if it existed). Once Fortunato learns that Amontillado is available for sale at a bargain price, he could easily find the seller for himself. No doubt a shipload of casks of Amontillado has just arrived in port--at least according to Montresor’s story). That ship (if it existed) would be easy for a man with Fortunato’s experience to find, and he could sample the wine aboard the ship and probably buy directly from the captain.
Montresor knows that Fortunato would think this way, because he has had plenty of dealings with the man in the past and has frequently been injured by him in business transactions. When Fortunato says, “Impossible!” he is only expressing his surprise that a shipload of gourmet wine should have arrived without his having heard about it. He assumes that he has missed out on this information because he has been drinking and carousing since the start of the carnival.
Montresor only entices Fortunato to his palazzo by telling him he is on his way to consult Luchesi. Fortunato doesn’t want Luchesi to hear about the shipload of Amontillado, because Luchesi would go searching for it on the waterfront himself. Then Fortunato would be competing with Luchesi in bargaining for the wine. If they bid the price up high enough, then Fortunato might be only able to purchase part of the cargo, while Luchesi got the other. Presumably either one of them would buy the entire shipload if they could get a bargain.
All three of these men would be interested in the Amontillado as merchandise on which to make a profit, and not for their own after-dinner consumption, although they might enjoy an occasional glass or two. Nobody needs 126 gallons of sherry for private consumption. The good thing about wine as merchandise is that it improves with age if it is contained in an oak barrel and thereby gains in value. So there would be no need for haste in retailing it.
Poor Montresor would have bought more than one cask if he had been sure of its quality, but he could not afford to buy the entire cargo of wine under any circumstances. At best he could buy another cask or two (if it existed!). So Fortunato decides to go with Montresor. The alternative, if Luchesi had not been mentioned, would have been to find the ship and sample the wine aboard it. But now what Fortunato is probably planning is to taste Montresor’s wine, shake his head, and tell him it is only ordinary sherry—then go looking for the ship, having eliminated both Montresor and Luchesi as potential competitors. And when Fortunato had beaten his competitors out of all the valuable Amontillado, he would laugh and call it “an excellent jest,” adding another injury to the thousand he has already committed against Montresor.
The existence of Luchesi enables Montresor to lure Fortunato into his vaults more easily than saying he just needs Fortunato's advice. Luchesi is a way for Montresor to appeal to Fortunato's pride. Fortunato considers himself an expert in fine wine and his pride is hurt when Montresor says he will consult Luchesi. Montresor knows that one of Fortunato's main faults is pride and he therefore uses this trait against Fortunanto by constantly mentioning a possible rival to Fortunato's skills.