“To illustrate the confusion ahead, experts gave the example of what would happen under the new regulations if someone attempted the interstate sale of a 100-year-old Steinway piano with ivory keys. Such a sale has long been permissible, because the piano qualified as an antique that contained ivory imported long before the mid-1970s, when officials began proscribing the material. But the new regulations would prohibit such a sale unless the owner could prove the ivory in the keys had entered the country through one of 13 American ports authorized to sanction ivory goods.”