The entire third paragraph of the narrative suggests that both Montresor and Fortunato are not noblemen but gentlemen who earn their livings by trading in expensive goods, which would include paintings, antiques, jewelry (gemmary), and gourmet wines. Venice, where the story is obviously set, has been a decaying city for centuries. Aristocratic families have been forced to sell off possessions, which could include masterpieces by great Renaissance artists, in order to survive. They would naturally want to deal with gentlemen-businessmen who were knowledgeable and discreet. In many cases, Montresor and Fortunato would not buy expensive items but would act as brokers and receive commissions. Many of the items to be sacrificed might never leave the seller's premises. The prospective purchasers would be brought to the seller's home while the family retired to another wing of their palazzo.
If Fortunato is a "quack" in some areas, this may partially explain why Montresor endures his "thousand injuries." Fortunato may call on Montresor to assist him in appraising such items as paintings and jewelry. The fact that Montresor considers Fortunato a "quack" suggests that Montresor has expertise in some things and Fortunato in others, such as wines, and that they have been interdependent on various occasions. Perhaps they have a symbiotic relationship—but Fortunato may take advantage of Montresor in business dealings whenever he can. The "thousand injuries" are likely injuries in business dealings. These are known only to Montresor. If it were widely known that Fortunato had injured Montresor many times, there would be some suspicion of Montresor when Fortunato disappeared. Montresor is anxious to avoid the slightest suspicion; he wants "impunity," i.e., no suspicion at all.
Fortunato appears to be richer than Montresor. This should also help to explain why Montresor puts up with Fortunato's "thousand injuries." Montresor can't afford to break off relations with this arrogant and selfish man. Montresor consistently acts obsequiously towards Fortunato. In one place in the narrative, Montresor reveals a great deal about the nature of their relationship.
"Come," I said, with decision, "we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi—"
Perhaps Montresor makes money by his relationship with Fortunato. He needs Fortunato more than Fortunato needs him. But then why kill Fortunato? Evidently Montresor has decided that he cannot put up with his behavior any longer. Besides, there may be some material advantage in eliminating a competitor.