What is the irony at the end of "The Gift of Magi"?

The irony of the ending of "The Gift of the Magi" is the fact that both Della and Jim unselfishly sacrificed their most prized possessions in order to purchase gifts that the other would be thrilled about receiving, but each of them sacrificed the very thing for which the other bought the gift. So, neither can use his/her Christmas present.
After counting her money--one dollar and eighty-seven cents--and realizing that she does not have enough for the gift which she wishes to purchase for her husband, Della is disconsolate. She wants to buy something

...just a little bit near to being worthy of the honor of being owned by Jim.

After she passes the pier glass, Della suddenly has an idea: She can sell her hair to Madame Sofronie. With this happy plan, Della hurries out while Jim is still gone. By the time Jim arrives, she has bought the gift worthy of him: a platinum fob chain for his gold watch that he inherited from his grandfather. Excitedly, Della uses the curling iron on her hair, makes the coffee, and anxiously awaits the return of her dear husband. In the meantime, Jim has purchased two gorgeous combs for Della's lovely tresses. These combs are made of beautiful tortoise shell with jeweled rims that Della has envied without hope of ever receiving them. When he arrives home, Jim, of course, is shocked to see that Della's hair is much too short for any combs. He just stares at her, but when she learns the reason, Della says encouragingly, "My hair grows so fast, Jim!" Then, she holds out the watch fob that seems to "flash with a reflection of her bright and ardent spirit." 

"Isn't it a dandy, Jim? I hunted all over town to find it. You'll have to look at the time a hundred times a day now. Give me your watch. I want to see how it looks on it."

Instead of being excited and obeying Della, Jim flings himself onto the couch, placing his hands behind his head as he tells her he sold his watch so he could buy her the beautiful combs. Ironically, now they cannot use the gifts they have received. Still, each is happy in the great gift of love from the other.