This is a great question. On the surface, the two gifts that were shared by Della and Jim, the two main characters, do not seem wise at all. Della cut and sold her hair to buy Jim a chain for his pocket watch, and Jim sold his pocket watch to buy Della a set of combs for her hair. So, in the end, they possessed gifts that they could not use. These might seem like unwise gifts, but these gifts were symbolically the best. Let me explain.
Della, in her poverty, gave her best. Jim, in his poverty, gave his best. In the end, this couple demonstrated incredible love. It was sacrificial. In other words, they outdid one another in love. In this regard, they were wise, because if it is more blessed to give than receive, they both gained something tremendous.