Is puberty sufficient for the validity of a minor's financial transactions, or is maturity also a condition?

The correct view is that a fifteen-year-old is an adult, not a child. The proof for this is what Ibn Umar narrated: that he was presented to the Prophet (peace be upon him) on the Day of Uhud when he was fourteen years old, and the Prophet did not permit him (to participate). Then he was presented to him on the Day of the Trench when he was fifteen years old, and he permitted him. The employee in a store is an agent for the owner of the shop, and it is not a condition for an agent to be mature (رشيد). Therefore, there is no harm in dealing with someone who is like that, and a person is not required to inquire about their status.