was fearful of losing a bargain."

Both men use the word "pipe" in referring to the Amontillado. A pipe contains 126 gallons. Fortunato does not seem like a sherry drinker at all. Sherry is a sipping wine, and Fortunato can chug-a-lug a whole quart of wine at a time. What interests him is the "bargain." If Montresor, who is a poor man, bought one pipe at a bargain price, Fortunato is thinking that he could buy the entire shipment and make a small fortune. 

Fortunato thinks Montresor trusts him and considers him his very good friend. Montresor knows that Fortunato is totally untrustworthy and that he is already planning on telling Montresor that the (nonexistent) wine is not genuine Amontillado, regardless of whether it is or not. Montresor only states that Fortunato is a true connoisseur because he knows that Fortunato would rely on his own judgment and would immediately buy up all the available pipes if he were satisfied that Montresor's is genuine. 

Montresor and Fortunato are not aristocrats but bourgeois gentlemen who deal in expensive merchandise. Montresor pretends to be in a big hurry to have an expert judge his Amontillado even though he has already paid for it and had it delivered to his vaults. The pretense of great urgency is to show Fortunato that he would like to buy more of the bargain wine if only he could be sure it is genuine. That is why Montresor pretends to be on his way to Luchesi. 

Fortunato does not have to taste Montresor's (nonexistent) wine. He knows the Amontillado would have had to be brought in by ship from Barcelona and that he would have no trouble finding the ship in the harbor and dealing directly with the captain, the purser, or whoever is selling it off at bargain prices. But if Fortunato were to try to put Montresor off by telling him he had an important engagement or that he was suffering from a bad cold, Montresor would immediately go to find Luchesi. Luchesi would probably do the same thing to the supposedly naive Montresor that Fortunato intends to do. Luchesi would tell Montresor that he could not assist him that night and then go looking for the nonexistent Spanish ship full of nonexistent Amontillado. So Fortunato has to go to Montresor's palazzo that night. He is greedy and unscrupulous and untrustworthy. He cheats people, even his friends, and thinks his dirty tricks are "excellent jests." Montresor is about to play an "excellent jest" on him.

Fortunato is easily persuaded by flattery, so much so that he fails to hear anything sinister in Montressor's voice or in his motives.  he wants to believe he is thought of as a great wine expert (the snobbiest, arguably, of all snobs) and the appeal to his specialized knowledge is the only leash Montressor needs to lead the unwitting prey to his doom.  

Edgar Allan Poe's short story "The Cask of Amontillado" is a tale of murder and revenge. The story depicts the revenge of Montresor against Fortunato for "the thousand injuries" he committed against him.

Montresor chooses to enact his revenge against Fortunato during Carnival. Carnival is a time, typically in February, in which people are able to celebrate excessively prior to lent. Many times, people go to extremes in partaking of alcohol and food. People also dress up and wear masks. They are able to be "something" or "someone" else during this period of celebration without worry of criticism.

Overall, Fortunato possesses two "traits" which make him an easy prey: being naive and being drunk. While both are not character traits, unless you consider Fortunato to be an alcoholic, each allow Montresor to lure Fortunato to his death.

Fortunato is drunk, given it is Carnival. He has a love of great wine- which is how one could justify naming him as an alcoholic. Since he is drunk, and a lover of great wines, Montresor is able to insure that Fortunato will follow him into the catacombs.

Second, Fortunato is naive. If he did enact a "thousand injuries" against Montresor, he should be more weary about Montresor. Instead, he trusts Montresor completely.

The character in Poe's "The Cask of Amontillado" with the ironical name of Fortunato is the possessor of personality traits that are, indeed, his nemesis.  In fact, he is easily duped because of these faults:

1. He imbibes so much that he is incapable of logical reasoning and clear perception.   When, for example, Montresor takes him farther and farther into the catacombs, the drunken Fortunato does not stop to wonder why they are proceeding so far.
2. He is vain, believing  himself a great connossieur of wine, one whose opinion should be valued over that of others. 
3. He is petty.  Fortunato does not want anyone to taste the Amontillado because he wants to be the one to judge it and take credit for having done so. Determined to not let his rival Luchesi taste the wine before he does, Fortunato becomes myopic in his thoughts as he risks his health in the damp catacombs filled with miter just to prevent Luchesi from tasting the Amontillado.
4. He is deluded in his judgment of Montresor, suspecting no ulterior motives from a competitor.  As a result, he is easy prey for his enemy who lures him deeper and deeper into the catacombs of his family.  When Montresor protests, Fortunato foolishly takes Montresor literally, saying that he is fine and there is no need to call Luchesi--"[he]is an ignormamus"--and is totally stunned when Montresor walls him in.

Fortunato was a known wine connoisseur and a "fake" connoisseur of many other things. Even Montresor gave him credit when it came to his knowledge of wine. Montresor used Fortunato's pride in his own intelligence and ability to get him to think that going to taste the amontillado was in fact Fortunato's idea. Montresor told Fortunato that he would ask Luchessi because he too knew about wine and Montresor didn't want to bother Fortunato. Montresor's idea to use reverse psychology on Fortunato proved to work. Fortunato played right into the trap because he knew that while Luchessi was knowledgeable in fine wine, he was no match for his own ability. This pride in knowing wine was ultimately the key that led to or was the result of his death. To put the situation in terms of cause and effect, the cause was Fortunato's pride and the effect was his eventual death later that same night.