What do Montresor and Fortunato have in common?

There are several strong indications that Montresor and Fortunato both earn their livings by dealing in things that appeal to wealthy buyers. Montresor mentions Austrian and British millionaires in the highly suggestive second paragraph of the story. Both these men refer to Montresor's imaginary Amontillado as a "pipe." This is a barrel containing 126 gallons. Montresor would never buy 126 gallons of a gourmet sherry for private consumption, and neither would Fortunato. That amounts to 500 quart bottles of Amontillado. Fortunato is only interested in it because Montresor says he got it at a bargain price. Obviously he must intend to bottle it and sell it off at a profit. He is anxious to get an expert opinion of the wine because he would like to buy more while he can still get a bargain. That is why Montresor pretends to be in such a hurry.

What these two men have in common is that they are sometimes competitors but often partners in business transactions. Many old Venetian aristocrats have to sell off family treasures in order to stay alive. They deal with men like Montresor and Fortunato who know the values of paintings, antiques, jewelry, and other one-of-a-kind luxury items and who know where to sell them. Montresor is a poor man and Fortunato is rich. Montresor must often ask Fortunato to go into partnerships with him, or borrow money from him, or collect finder's fees from him. No doubt the "thousand injuries" Montresor has suffered have been in business dealings. This would explain why Montresor maintains relations with Fortunato although he hates him. It could also explain why nobody knows about these thousand injuries. If people knew Fortunato had injured Montresor so many times, then Montresor would become a suspect after Fortunato disappeared--and Montresor wants to be above suspicion. So he continues to act as if Fortunato is his best friend, and he has conditioned himself always to refer to Fortunato as "my friend," "my best friend," "my good friend," etc., as he does many times throughout the story.

There is a symbiotic relationship between these two men. Montresor needs Fortunato for making money. Fortunato gets tips on potentially profitable deals from Montresor. For example, Montresor might know that a certain Venetian nobleman would like to sell an oil painting by an Italian Renaissance master. If the nobleman wants cash rather than having Montresor act as a broker, then Montresor might refer the nobleman to Fortunato, who could come up with the cash and resell the painting for a big profit. Montresor would expect a finder's fee, but he might often get a smaller fee than he thinks he deserves--or in some cases he might not get any fee at all. Fortunato wears a jester's costume during the carnival because he considers himself a clever jester. He may pull many dirty tricks on Montresor and laugh them off as clever jests. 

Both Montresor and Fortunato are connoisseurs. The sellers and buyers trust them to know about such things as authenticity and especially values. Money is the all-important factor in these men's relationship.