He had a weak point -- this Fortunato -- although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity to practise imposture upon the British and Austrian MILLIONAIRES. In painting and gemmary, Fortunato, like his countrymen , was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially; I was skilful in the Italian vintages myself, and bought largely whenever I could.
This is the third paragraph of "The Cask of Amontillado" quoted in full. It conveys two main ideas. One is that Montresor is not an Italian but a Frenchman. He distances himself from the Italians by his disparagement of their judgment of "painting and gemmary." His family may have lived in Venice for several centuries, but he is still an outsider as far as the upper-class Italians are concerned. His catacombs may be full of human bones--but these are not necessarily all bones of his ancestors. He may not have cleared out the bones that did not belong to his family because there was no place to put them; or there may be some law against such removal. In fact, it is quite possible that Montresor doesn't even own his palazzo but is renting it. He is obviously not affluent. He says that he bought largely (of Italian wines) "whenever I could." That should be interpreted to mean whenever he could affordto. There would always be plenty of Italian wines available.
The other main idea conveyed in Poe's third paragraph is that both Montresor and Fortunato are specialists in luxury goods and must both be earning their livings through buying and selling expensive merchandise to wealthy people. They are in the same line of business, but they are also competitors. While Montresor is poor, Fortunato is rich. It is very likely that the "thousand injuries" which Montresor does not explain are injuries suffered in business deals. Fortunato can outbid him. Fortunato can afford to buy in larger quantities. And Fortunato has family connections going back for over a thousand years. Italians would favor him because of his family status. If there is something good to be bought at a bargain price, Fortunato is more likely to hear about it before Montresor.
When Montresor tells Fortunato that he has bought a cask of Amontillado, Fortunato says, "Impossible!" What he really means is that it would be impossible for Montresor to learn about such a cargo of valluable wine before he did. But this is carnival season and Fortunato has been drinking and not attending to business. He thinks this is why Montresor has gotten ahead of him. However, Montresor has only bought one "pipe" (126 gallons) because, as he says, "I have my doubts" (about the genuineness of the wine). Fortunato is highly motivated to taste it--not because he needs any more wine, not because he is anxious to show off his connoisseurship, not to accommodate a friend--but because he wants to buy some of the Amontillado himself for resale. But he himself must taste it to make sure it is genuine, since Montresor has repeatedly expressed his doubts. Fortunato can afford to buy the whole cargo and make a big profit--and Montresor knows that is exactly what Fortunato is planning to do because that is exactly the sort of injurious thing Fortunato has done in the past.
Montresor tells Fortunato that he has bought some wine that he thinks is Amontillado, but he needs an expert opinion to tell him if it is genuine. Montresor flatters Fortunato in order to get him down in the catacombs. He knows Fortunato considers himself an expert of wines, so he uses that to lure him to his death.