i think
they were selling what they didn't have at a highly-inflated price, knowing that said price would drop rapidly, with traders desperate to offload their shares once they knew the orange crop was fine. this allowed them to buy shares extremely cheaply, which they could then give to their earlier buyers, safe in the knowledge that the buyers had paid well over the odds, leaving them with a very healthy profit.
( Smash Monkey lowering the tone of the whole internet, Fri 14 Oct 2011, 12:16, 1 reply)

You are not alone in your confusion:

www.wisebread.com/explaining-the-climax-scene-of-trading-places

Also, it was only when typing 'How does the share trading scam at the end of trading places work?' that it dawned on my that it's called Trading Places not just because the people trade places but that it's also set around a trading place...

That would almost qualify as an answer to the question if I had actually been mystified by it as opposed to just utterly unaware.
( scarpe We Stole Bikes, Fri 14 Oct 2011, 12:19, closed)

i was right!
this pleases me.
( Smash Monkey lowering the tone of the whole internet, Fri 14 Oct 2011, 12:25, closed)

Having read that,
I stand by my earlier comment.

I must confess to not twigging the dual meaning of the film's title, though.
( monster munch person, man, woman... camera... TV?, Fri 14 Oct 2011, 12:26, closed)