Gains From Trade

In economics, trade is considered the voluntary exchange of goods and services. People who want to trade with others must be willing to give something up in order to get the item they're looking for. People will only participate in trade voluntarily when they expect to gain from the exchange. 

The following scene from Moneyball shows Billy Beane and Peter Brand contacting a number of other General Managers in Major League Baseball in an effort to identify other players they may be willing to trade. Beyond just exchanging players, teams are also willing to exchange future draft picks or cash in order to get (or get rid of) a player they (don't) want:

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