It is significant that Montresor claims to have bought such a large quantity of Amontillado. It seems unlikely that he would have wanted that much of that kind of wine for himself. A pipe containing 126 gallons would make 500 quarts. More likely Montresor would bottle it and sell it in cases at a good profit--that is, if the Amontillado truly existed. Fortunato is not interested in merely tasting the wine and displaying his conneisseurship. He would like to buy some of that bargain-priced Amontillado himself. But he has to taste it to make sure it is genuine, because Montresor tells him he has doubts.
"I have my doubts," I replied, "and I was silly enough to pay the full Amontillado price without consulting you in the matter. You were not to be found, and I was fearful of losing a bargain."
Note that he says it was a bargain. Fortunato is rich. He could buy a whole shipload of Amontillado and strike an even better bargain. This is why he is so eager to taste Montresor's wine.
More specifically, a 'pipe' is a wine cask having a capacity of 126 gallons or 2 hogsheads (478 liters).
One evening, during the Carnival season the narrator Montresor meets Fortunato and lures him into his cellars by lying to him saying that he had received a cask of a rare wine called Amontillado and that he would like Fortunato to certify its genuineness:
It was about dusk, one evening during the supreme madness of the carnival season, that I encountered my friend......I said to him -- "My dear Fortunato, you are luckily met. How remarkably well you are looking to-day! But I have received a pipe of what passes for Amontillado, and I have my doubts."
A pipe is a large barrel of wine. You are probably referring to when he says he received "a pipe of Amontillado." Montresor is telling Fortunato that he has a large amount (a "pipe") of a rare kind of wine in order to seduce him into the cellar.