Legal texts discuss those same sorts of transactions under the heading, "contract law." But contract law represents only one of many jurisprudential concerns. Students of the law also spend a great deal of time studying other sorts of transactions—transactions marked by implied consent, hypothetical consent, or varieties of non-consent.
Perhaps the law takes too dark a view of human nature. Or perhaps economics takes too bright of one. Non-expressly consensual transactions do happen, after all. Even if rare, they can powerfully affect—or even end—our lives. But non-expressly consensual transactions are, in fact, not rare. They pervade our world.
Consider government, a service supplied and demanded subject to varying levels of consent. A consumer might demand government services expressly, impliedly, hypothetically, or in some mixture thereof. An immigrant might, for instance, swear allegiance to the U.S (showing express consent) after having taken up residence in it (showing implied consent).
Consumers can object to government services in similar fashion, denying consent expressly, impliedly, or hypothetically, in measures unique or in combination. Thus, for instance, an emigrant might disavow allegiance (expressly not consenting) to and flee (signaling implied non-consent) from a tyrannical (and therefore hypothetically non-consensual) regime.
Combine these observations, and you can picture consent and non-consent on a scale running from most to least:
c + express
o + implied
n + hypothetical
e - hypothetical
n - implied
t - express
As that scale of consent indicates, different types of consent have different probative values. Express consent conveys more information about a transaction than implied consent does. Thus, for instance, an agreement's written terms trump those merely assumed to apply by dint of customary practices. (See, R. (2d) Contracts § 203(b).) Implied consent, in turn, says more than hypothetical consent does. What people actually do says more about their preferences than mere theory does. Non-consent, too, comes in different strengths, running from weak (hypothetical non-consent) to strong (express non-consent). "No!" thus tells us more about whether you agree to a proposition than if you silently turn away, and both signal your views more reliably than our musing on what you would agree to.
Many interesting results follow from this model of consent. I explored some of them in a paper I wrote under the tutelage of Judge Richard Posner, The Jurisprudence Of Polycentric Law (unpublished manuscript, 1992). In particular, I there describe express consent "as an ideal standard for ranking surrogates such as implied and hypothetical consent. The nearer these substitutes come to obtaining a person's express consent, the better they justify the legal obligations they serve to defend."
Whether and to what extent that approach to justification jibes with Randy Barnett's poses an interesting question, one that I plan to address later and at length. First, though, I plan to tackle another application of the scalar consent model: I plan to use it to describe the supply and demand of government services. More on that project, anon.