In "The Gift of the Magi," Jim and Della are representative of the Magi. The Magi are the three kings that visited the Christ child and gave the ultimate gifts as far as giving is concerned.
Jim and Della are considered the Magi because they gave each other the ultimate gift. Della sold her most prized possession--her hair--to buy Jim a platinum chain for his watch. Jim sold his most prized possession--his watch--to buy Della precious combs for her beautiful, long hair.
In this type of giving, the author considers Jim and Della the Magi. They made the ultimate sacrifice one for the other. No one has given more. Jim and Della are considered the Magi because they sacrificed their most prized possessions one for the other.
Della planned to make her gift worthy of someone like Jim:
"Many a happy hour she had spent planning for something nice for him. Something fine and rare and sterling—something just a little bit near to being worthy of the honour of being owned by Jim."
Selling her hair is the ultimate sacrifice. Jim loved Della's long hair. Della parted with her hair in order to give Jim a worthy gift. This giving compares to the giving of the Magi to the Christ child. No greater giving is recorded.
Jim values his wife more than he values his watch. His love for his wife goes beyond materialism. Jim clearly values his young wife more than his gold watch. He sells his watch to buy her a set of beautiful, jewel-edged tortoiseshell combs for her long hair.
Jim honors his wife above his own happiness. He gives the ultimate gift when he sells his watch to buy her the combs for her beautiful hair. As the author states, Jim and Della are the Magi:
But in a last word to the wise of these days let it be said that of all who give gifts these two were the wisest. Of all who give and receive gifts, such as they are wisest. Everywhere they are wisest. They are the magi.