Wis. Stat. § 786.52: When sale not avoided.

A sale of real estate by a guardian is not avoided by any irregularity in the proceedings if it appears that the guardian was licensed to make the sale by the circuit court having jurisdiction; that the guardian gave a bond which was approved by the court before the sale if a bond was required; that the guardian gave the notice of the time and place of sale as prescribed by law; that the premises were sold accordingly and the sale confirmed by the court, and that the premises are held by one who purchased them in good faith.