Any entered or unentered merchandise (except merchandise entered under section
1557 of this title, but including merchandise entered for transportation in bond or for exportation) which shall remain in a bonded warehouse pursuant to section
1490 of this title for 6 months from the date of importation thereof, without all estimated duties, taxes, fees, interest, storage, or other charges thereon having been paid, shall be considered unclaimed and abandoned to the Government and shall be appraised and sold by the Customs Service at public auction under such regulations as the Secretary of the Treasury shall prescribe. All gunpowder and other explosive substances and merchandise liable to depreciation in value by damage, leakage, or other cause to such extent that the proceeds of sale thereof may be insufficient to pay the duties, taxes, fees, interest, storage, and other charges, if permitted to remain in pursuant to section
1490 of this title in a bonded warehouse for 6 months, may be sold forthwith, under such regulations as the Secretary of the Treasury may prescribe. Merchandise subject to sale hereunder or under section
1559 of this title may be entered or withdrawn for consumption at any time prior to such sale upon payment of all duties, taxes, fees, interest, storage, and other charges, and expenses that may have accrued thereon, but such merchandise after becoming subject to sale may not be exported prior to sale without the payment of such duties, taxes, fees, interest, charges, and expenses nor may it be entered for warehouse. The computation of duties, taxes, interest, and fees for the purposes of this section and sections
1493 and
1559 of this title shall be at the rate or rates applicable at the time the merchandise becomes subject to sale.