Della looks at her hair with brilliantly shining eyes because it has just occurred to her that she could sell it to buy an expensive Christmas gift for Jim. Just before she glimpses it in the mirror, she is wondering what she can buy for him with the mere $1.87 she has saved.
We note too that Della is conflicted about the sale of her hair. While her eyes are shining at the idea of selling it for Jim, her "face had lost its color within twenty seconds." This is because she also realizes that she loves her hair. It is very long, rippling down to below her knees. She thinks the if queen of Sheba lived across the street, she would hang her hair out the window to put the queen's jewels to shame.
Della loves Jim, which is why her eyes light up at the idea she can use her hair to buy him a gift. But Della loves her hair, which is why her face pales at the thought of sacrificing her hair. Selling her hair is hard for her to do. Nevertheless, her love for Jim will win the day.