Before a license for the sale of any property of the minor shall be granted, the guardian of the estate shall take the following oath and file a certificate of the oath with the court:
"I, __________ , guardian of the estate of __________ , a minor, do solemnly swear that in disposing of the estate of the minor, for which I have applied for license, I will use my best care and judgment, exercise the prudence, intelligence and discretion which I would in the management of my own affairs and will act in the best interests of the minor and the minor's estate so that property shall be sold to the utmost advantage to the minor or the minor's estate, without any self-interest whatever."