The answer to this question is a simple one. A "pipe," also known as a "butt," was an old unit of measurement, used to describe the size of a large wine cask. A pipe contained just under 500 liters, or 125 US gallons, a substantial amount of wine. So when Montresor claims that he has purchased a "pipe" of Amontillado, he has made a significant purchase. It is thus plausible that he might want to get a wine expert to verify its authenticity, and he is able to play on Fortunato's vanity by asking him to sample it. This, of course, is how he lures the doomed man into his cellar. In fact, Fortunato's only doubt is that Montresor could have obtained such a large quantity of wine:

“How?” said he. “Amontillado, A pipe? Impossible! And in the middle of the carnival!”

Montresor bolsters his ruse by agreeing with Fortunato, suggesting that, on second thought, he too wonders whether his purchase was not too good to be true.

What does "pipe" mean in Poe's "The Cask of Amontillado"?

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.