Answer:
What is bought by the yard by is worn by the foot?
Not born, but from a Mother's body drawn. I hang until half of me is gone. I sleep in a cave until I grow old. Then valued for my hardened gold.
My teeth are sharp, my back is straight, to cut things up it is my fate.
Always wax, yet always wane: I melt, succumbed to the flame. Lighting darkness, with fate unblest, I soon devolve to shapeless mess.
A box without hinges, lock or key, yet golden treasure lies within.
You use it between your head and your toes, the more it works the thinner it grows.