In the book both Jim and Della want to buy gifts for each other but can't afford them because they are poor. This means there are going to be some sacrifices to be made in order to buy each other gifts.
It's Christmas and I can't afford to buy my spouse a gift.
I sold my watch to buy you these combs.
The theme in the story was gratitude because Jim and Della were willing to give up their most prized possessions to buy each other gifts. Unfortunately, the gifts had a lot to do with their most prized possessions.
I sold my hair to buy you this watch chain.
In this scene Della is going to cut her hair, which is her most prized possession, and is selling it to buy Jim a gift. Jim does something similar to this by selling his watch to buy Della a gift. This symbolizes sacrificial love
I'm selling my glorious hair to buy my husband a gift.