Any entity can be marketed. The limitations of what actually is marketed, therefore, are imposed solely by motivation, which is to say that the entities which are marketed are those which someone has found worth marketing, in commercial, political, or social terms.
Marketing which is directly commercial in nature, aimed at persuading the consumer to buy a product, is actually much more recent than, for instance, political marketing, which can clearly be seen in Greek and Roman politics. Even in the eighteenth century, commercial advertising was still very basic. An advertisement in a newspaper would often say something like "J. Anderson, Butcher. Andover, Mass." This simply let consumers know that if they wanted to buy meat in Andover, there was a place where they could do so. It was not marketing in the sense of persuasion.
Twenty-first century branding, however, means that different types of entities are often now marketed together, with one that is already popular lending credibility to the one which is for sale. The most obvious instance of this is the celebrity endorsement. However, in the 1990s, it became more common to try to associate a popular idea or movement with the brand, rather than an actual person, whose image might later be tainted. A prime example of this marketing strategy is Benetton, which attempts to market its products alongside the idea of being antiracist.