O. Henry himself writes at the end of "The Gift of the Magi":
And here I have lamely related to you the uneventful chronicle of two foolish children in a flat who most unwisely sacrificed for each other the greatest treasures of their house.
So according to their creator, Jim and Della were "foolish" and "unwisely" decided to sacrifice their most treasured possessions. But O. Henry suggests that we do a lot of foolish and unwise things for people we love. This is the way we feel about Jim and Della at the end. We feel sorry for them. We feel they are terribly young and naive. Della may recover her hair in time, but Jim can never recover his watch, and the watch was a treasured family heirloom which he should have kept out of respect for his father and grandfather. The story ends in a disaster for these dumb young kids. Nevertheless, they are to be forgiven and even honored because they loved each other so much that, out of desperation, they made such heroic sacrifices to buy each other Christmas presents which, as it turned out, neither of them could use.
How many Christmas presents are given and received each year which are chosen by mistake or received by people who don't like them but have to pretend to be delighted? O. Henry is suggesting that it is not the Christmas present that is important but the affection it represents. The love that Jim and Della have for each other is worth, to use O. Henry's imagery, more than the treasures of the Queen of Sheba or King Solomon.
A platinum watch fob is a treasure even without the watch. A set of beautiful hair combs is a treasure even without the hair. Jim can fondle his watch fob and Della can even try out her combs on the remains of her hair. And for each of them the presents will be symbols of thoughtfulness and love.