What did Della sell to get the $20.00 she needed to buy her husband's present?

Della had beautiful brown hair. Most women in her day let their hair grow very long and had to pin it up in a sort of bouffant style. Della was proud not only of the rich quality of her hair but of its length. O. Henry describes the length as follows:

So now Della's beautiful hair fell about her, rippling and shining like a cascade of brown waters. It reached below her knee and made itself almost a garment for her. 

She must have spent many hours caring for her hair. Washing it would be a big project, and then letting it dry would take a long time because there were no hair dryers in those days. But most women spent their time at home except when they went shopping. There was a common saying that "A woman's place is in the home." Women also wore dresses that covered them from their necks down to their ankles and had sleeves that covered their arms down to their wrists. The hair and the clothing restricted their movements and seemed to symbolize their domestic confinement and servitude.

Della makes a great sacrifice when she sells her hair. O. Henry describes the transaction in a way intended to show how she must have been feeling. The woman who buys her hair is inconsiderate and rude. Madame Sofronie can turn on her fake Old World charm with people who buy her wigs and toupees, but she makes up for it when she is dealing with hair sellers rather than hair buyers. 

O. Henry spends many words describing Della's beautiful long hair in order to emphasize the stark contrast after she has been scalped. She has traded one problem for another. She wanted to buy Jim a present, but now she is afraid she will lose his love when he sees her.

Within forty minutes her head was covered with tiny, close-lying curls that made her look wonderfully like a truant schoolboy. She looked at her reflection in the mirror long, carefully, and critically. 

It turns out that Jim has also made a sacrifice. He has sold his treasured gold watch. But this is Della's story. Jim is only a minor character who comes in at the end. People will always remember "The Gift of the Magi" as a story about a young woman who sold her hair to buy her husband a present for Christmas.