Subject to the provisions of this Act, the unpaid seller's right of lien or
stoppage in transitu is not affected by any sale or other disposition of the
goods which the buyer may have made unless the seller has assented thereto:
Provided that where a document of title to goods has been lawfully transferred
to any person as buyer or owner of the goods, and that person transfers the
document to a person who takes the document in good faith and for valuable
consideration, then if such last-mentioned transfer was by way of sale the
unpaid seller's right of lien or stoppage in transitu is defeated, and if such
last-mentioned transfer was by way of pledge or other disposition for value
the unpaid seller's right of lien or stoppage in transitu can only be
exercised subject to the rights of the transferee.