Mode of sale discretion: commissioners may order auction when taxpayer's recalcitrant conduct seeks to delay recovery. Commissioners retain discretion over the mode of sale of property for recovery of tax arrears and should exercise caution before allowing negotiated sale where the assessee's conduct causes repeated postponements; if the Commissioner considers the assessee recalcitrant, he may proceed to auction without offering an opportunity for negotiated sale to prevent delay and disruption of recovery.