Jim and Della were very impractical in selling their most valuable possessions, Jim's heirloom pocket watch and Della's fabulously long hair, in order to raise money to buy Christmas presents for each other. They were financially poor; if they wanted to sell their things, they could have used the funds for bettering their living situation.
The original magi were the wise men who brought exotic gifts to the Baby Jesus. A poor baby, born to unmarried parents in an isolated village, would have had no practical way to benefit from receiving frankincense, gold, and myrrh.
In the end, Jim and Della's gifts come to be regarded as being the more valuable gifts because they demonstrated the great depth of their love and commitment to each other. With time, they would be able to utilize the gifts, and they could enjoy the anticipation of that time along with their affection.
"The gift of the magi" was the gift of love, given deeply and freely by Jim and Della to demonstrate their devotion and dedication.