As in this story, On first Della telling details about setting that hint at plot. In the first paragraph, she open it up by introducing herself and money problem.
Della sells her hair to buy a watch chain for Jim's pocket watch, but Jim sells his watch to buy Della beautiful combs for her long, luxurious hair. In this case, each has sacrificed what was most dear to him or her for the other—which the other then cannot use.