Plumbing is not entirely a zero sum game, and referrals are a big part of exploiting division of labor. If two plumbers are good at different aspects of plumbing, giving referrals to each other when they're called to jobs for which the other one is better suited has positive dividends - better plumbing gets done and customers are happier. A plumber passing on work this way is passing up the immediate benefit of a contract in hand, but presumably trades that for a better contract in the future that he might otherwise not have gotten, not for nothing at all.
Sure, yes, coalitions form in even unlikely places. But even so, come on. Plumbing is largely a zero sum game, or at least far more so than book sales.
Perhaps, but it's not relevant to the situation I described, which relied on each plumber having an absolute advantage over the other one. For relative advantage to come into play, there would have to be some kind of limit to the total amount of work one plumber could do.
