One of the moral lessons of "The Gift of the Magi" is that we often don't value what we truly should. Jim loves Della, her hair included, and he doesn't need a fancy watch chain for Christmas to make him happy--he already has a wife he adores. Della, for her part, loves Jim and doesn't need hair combs to make her happy. In other words, they already have what truly makes them happy--each other--and the Christmas presents are secondary. The sacrifices they make for each other, as Della sells her hair and Jim sells his watch, show how much they love each other and show the sacrifices they are willing to make for each other. This love and willingness to sacrifice what means the most to them for the sake of the other person are their true presents, not material goods.
One moral of this story is that we will give up possessions that mean a great deal to us because of love for another. Another moral is that happiness does not depend on money, but on love. Still another is that the true spirit of Christmas is that of giving, not of getting. Of course, the author, who is famous for ironic twists in his stories, has structured the story so that each has sacrificed for the other in a way that makes each gift useless to the other, showing us that the sacrifice is the demonstration of love, giving, and the spirit of Christmas, not the "value" of the item purchased.