To decide how to buy or sell a private good is a task for an individual in the standard economics model. It would be better to amend even this simple case because the terms of a trade are seldom left entirely in the hands of the individual traders involved. Haggling about the terms for a single trade is uncommon in most economies today. Yet it is the staple of beginning economics courses. It portrays an individual free to buy a bundle of commodities, each available at a given price and in any amount at the pleasure of the buyer. A student is told consumers choose what to buy in order to maximize their utility subject to a budget constraint. Any objection to this picture raised by a student is typically brushed aside by saying it is a model not a realistic picture of consumer behavior. Evidence to test the model rarely comes up.