"Fiduciary" is a legal term of art, and can easily be used a proxy for all of the other statutory or contractual, or common law duties you specified. When you are talking about a multimillion-dollar art purchase, I should think you'd want your art agent to have a fiduciary responsibility to act solely for your benefit and in your best interests, rather than rely on a particular forum's law. But, again, it's a matter of taste, really. How much layering of legal protection do you want to put into a contract?