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How are they earning money in the movie “Trading Places”?
They are not selling stocks. They are selling OJ futures contracts. Selling a futures contract at 142 gives the buyer the right to buy a fixed number of pounds of orange juice concentrate ("OJ") on a future date at 142 cents per pound. The seller has an obligation to suppy that fixed number of pounds of OJ to the buyer on the future date for 142 cents per pound. When the seller turns around and buys future contracts at 29, the seller gets the right to buy OJ on a future date at 29. This "zeros his position" -- meaning he's guaranteed himself the ability to deliver the pounds of OJ he was obligated to supply when he sold futures contracts at 142. And since he'll only have to pay 29 cents per pound, and he'll be selling the OJ for 142 per pound, he'll walk away with 113 cents of profit for every pound sold. You can read a blow-by-blow account of what Winthorpe and Valentine did at the end of "Trading Places" here and here. Note that what they did would not be legal today under the "Eddie Murphy rule", which prohibits trades based on illicitly obtained government information.