There is an interesting story narrating the belief behind buying valuables for prosperity and happiness of family on dhanteras.
The story of Naraka Chaturdashi:
An ancient legend imputes the occasion to an intriguing tale about the 16 year old son of King Hima.
His horoscope predicted his death by snake-bite on the fourth day of his marriage. On that particular day, his newly-wed wife did not allow him to sleep. She laid out all her ornaments and lots of gold and silver coins in a heap at the entrance of the sleeping chamber and lit lamps all over the place. Then she narrated stories and sang songs to keep her husband from falling asleep.
The next day, when Yama, the god of Death, arrived at the prince’s doorstep in the guise of a Serpent, his eyes were dazzled and blinded by the brilliance of the lamps and the jewelry. Yam could not enter the Prince’s chamber, so he climbed on top of the heap of gold coins and sat there the entire night listening to the stories and songs. In the morning, he silently went away.
Thus, the young prince was saved from the clutches of death by the cleverness of his new bride using the jewelry and valuables, and the day came to be celebrated as Dhanteras.
Other related folk story:
Another legend says, in the cosmic battle between the gods and the demons when both churned the ocean for ‘amrit’ or divine nectar, Dhanavantri – the physician of the gods and an incarnation of Vishnu – emerged carrying a pot of the elixir. So, according to this mythological tale, the word Dhanteras comes from the name Dhanavantri, the divine doctor.