It says the two FOOLISH people UNWISELY sacrificed their treasure. But it says after that, these gifts were the WISEST. Why are they the wisest? From The Gift of the Magi
The title The Gift of the Magi proves important to your question. The Magi who brought gifts to Jesus couldn't possibly know how important their demonstration of recognition for a savior was at the time.
Both Jim and Della gave literally the most important thing that they owned to each other. Their sacrifice was so tremendous that the outcome of the story proves that the gift wasn't what mattered, it was the gesture to be willing to risk all, or at least that material item at the time which meant the most to them. Likewise, I think this proves that items are not important at all, relationships are.
To me, the gifts that Jim and Della give each other are the wisest because they are the gift of love.
The actual things that the two of them give each other are not that important. What really is important is what they each gave up for the other. Each of the two of them sacrificed something that was tremendously valuable to them in order to try to make the other happy. This is a pretty good way to prove that they love each other. Getting such proof of someone else's love for you would be a wonderful gift.