What character traits make Fortunato such easy prey for Montresor?

Fortunato is avaricious, unscrupulous and deceitful. Montresor knows his victim thoroughly. He knows that Fortunato will be tempted by the prospect of making money off the nonexistent Amontillado if he tells his intended victim that he bought it at a bargain price.

"You were not to be found, and I 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.