Parent company Vornado Realty Trust has been dropping hints along Wall Street and with potential buyers that it wants to sell the subsidiary, including the fortress-like structure along the Chicago River. The New York real estate investment trust purchased it from the Kennedy family in 1998 for $630 million. Vornado's efforts already have yielded a $1.25-billion offer, that deal died last month, people familiar with the negotiations say. A sale could portend big changes for Mr. Kennedy, 47, who as Mart president since 2000 has led a major expansion of the business, which stages trade shows and leases showroom...