Trading Places Ending Explained

From Mah Two Cents:

They were selling high and buying low? And they got away rich? Gah-huh?
Like I said, I thought that, too. Okay, as explained to me and I figured out: Dan Ackroyd and Eddie Murphy wanted to get back at the Duke Brothers (Don Amechie and Ralph Bellamy) for making them trade places. They learned they were getting the orange futures from an industrial spy (Paul Gleason, you know, that prick teacher from Breakfast Club) So they intercept the report that says the frost won’t affect the orange crop and give the Dukes a fake saying the opposite, I assume. As explained to me, in the futures market you can buy shares from the exchange (and I assume on margin, or a fraction of the cost, with the full amount called in certain circumstances as we’ll get to later. And key to this transaction, at the end of the day, they have to give back the options to get their money paid for same.) So our heroes take Ophelia’s and the butler’s money, get all the shares they can, and are ready. Our heroes sell their futures at around 29, then the report comes on saying the orange futures aren’t affected by the frost, and buy back around 131. The Dukes realize what’s going on, but too late to take action, their margins are called and told their seats would be taken if they couldn’t pay. As pointed out to me in that explanation, that’s probably why all 4 of our heroes took off to the Caribbean to escape the inquiry that would most certainly rise as a result.