Two honest traders can both want a deal and still be unable to reach one
Roger Myerson and Mark Satterthwaite proved in 1983 that when a buyer and a seller each privately know their own value for a good, no trading mechanism can simultaneously be efficient, voluntary, and need no outside subsidy. Even when a mutually beneficial sale clearly exists somewhere in the gap between what each would accept, hidden information alone can doom every fair way of striking it.