Thats an extremely restrictive definition of zero sum. If that were the normal definition most transactions would be considered zero sum.

But its not. Zero sum has to do with the utility provided to each side of the transaction. I may have a completely different desire to own a future than you. Perhaps my portfolio needs less risk or my predicted oil needs have changed. In those cases I gain a lot of utility by being able to move out of the future into cash, even if later you make more than I would have by selling it.