Where the goods sold under any home solicitation contract are so affixed to real property as to become a part thereof, whether or not severable therefrom, the buyer shall not have the right to cancel as provided in Section 1689.6 or Section 1689.7 if, subsequent to his signing such contract, he has sold or encumbered such real property to a bona fide purchaser or encumbrancer who was not a party to such sale of goods or to any loan agreement in connection therewith.