Your comment reduces my confidence that I understand the term "externality". Until I read it, I tentatively believed that "X has positive externalities" means that X is an action taken voluntarily by a person (or firm) and has positive expected global utility. Most economic discourse assumes that all voluntary actions taken by a person (firm) have positive expected personal (organizational) utility. But in the present environment, counterfeiting money has according to my models negative global expected utility by reducing (by a small amount) the value of every asset denominated in the currency being counterfeited (e.g., cash and loans). (Counterfeiting is a member of the class or set of a diffuse harms, which by the way do not seem to get the attention they deserve here on Less Wrong.)

(Buying junk I do not want has negative global expected utility, too, under my models.)