Right, you had a question. You're fine to sell your image of the Willis (nee Sears) Tower as a postcard. The registration that you mentioned is not for the image of the building -- it is only for the words "Willis Tower" and it is only for use with real estate services. The Dear Rich Staff could find no trademark registration for the image or design of the tower (although a registration is not necessary to claim trademark rights). Ennyway, as we mentioned in a previous post, in order for someone claiming trademark rights in the building design to stop you, the following would have to be true: (1) the building would have to have an identifiable, distinctive appearance; (2) the building would have to be publicly associated with certain goods or services; (3) your use would have to be commercial (not editorial); and (4) your use would have to be linked to an offer or endorsement of similar goods or services.Based on that analysis, you should be good to go.
And speaking of photographs of buildings. Check this out.