Not so much depends on shrewd bargaining, on bluff, or on the stubbornness of an individual. Far more depends on forces outside the control of any one man. The bidders are impelled by self-interest to outbid their competitors, and thus the limits within which the market price must fall are narrowly fixed. [Sidenote: Buyers fix price of perishable goods] If things already brought to market must be sold at any price that can be secured, the buyers may be said to fix the price. This does not mean that they can buy it for any sum that they wish, but it means that when each one is trying to get it as cheap as possible, their bids finally determine how much it will sell for. In such cases, therefore, the competition is for the moment one-sided. If a part of the supply can be withdrawn and kept without great loss, this will be done if the price is low. Strawberries, fish, and meat may be sold Saturday night at any price that will secure purchasers, but every thing that can be kept with little or no depreciation will be withheld from sale for a time. It may even be of advantage to the seller to destroy a part of the supply, when the increased price of the smaller amount will give a larger total. [Sidenote: The margin of advantage and the marginal pair] 3. _Where two-sided competition exists, the bidding goes on until a price is reached where the least eager seller and the least eager buyer have the narrowest possible motive to exchange_. As the market ratio varies from those in the minds of the individuals when they come to the market, there is left a considerable margin to some and a very small one to others. This difference between the market value and the ratio of exchange at which any given individual would continue to exchange for the good may be called the _margin of advantage_. Moreover, the buyers will have a margin and the sellers a margin, and as that margin narrows there is less and less motive to continue the exchange until, finally, the margin disappearing, the buyer or seller, withdrawing from the market, ceases to be an exchanger, at least for that particular part of the goods. The least eager buyer and the least eager seller may be called the _marginal pair_. They are the buyer and the seller respectively having the narrowest margin of advantage. Their outside estimates are nearest to the market ratio. If the market ratio shifts slightly in either direction, one of them will drop out of the exchange. It is evident that a buyer who is taking ten units may be on the margin with reference to the tenth unit, and yet may continue to be one of the most eager buyers to secure one unit. Thus, the marginal buyer is to be thought of as that person who, logically considered, is the least eager, or on the margin, with reference to a particular unit of supply, however eager he may be with reference to any other unit of supply. It would be well to recall here the discussion of the nature of wants and the variation in the intensity of demand.