Bananas and oranges go by nightfall after arrival in the day by ship, and there are no other sources of bananas and oranges. For every banana a person eats, they must also eat an orange.
Ships arrive sporadically throughout the day, and a ship can only contain bananas or oranges, not some of each. Only a few ships arrive each day. An equal number of bananas an oranges are shipped each day from the sources, but some of the ships sink on the way, so that there may be a surplus or deficit of bananas or oranges (remember, to eat one banana you must eat one orange).
Spotters see the ships coming at roughly the same time, with some randomly variation, and immediately adjust the price of existing bananas or oranges based on what the incoming ship is carrying--after all it didn't sink, and the fact that it didn't gives some information on whether or not there will be a banana/orange deficit.
One day a man invents a telescope and begins seeing the arriving ships five minutes before everyone else. He provides no net-wealth to society as a whole, but with the money he makes via arbitrage he begins commanding a disproportionate share of society's wealth for his own use.
*Galileo originally used his telescope invention for exactly this (in conjunction with a wealthy family)--one of the least useful things he ever did. Would you rather someone work in HFT (scope out banana boats 5 minutes earlier than the next guy) or work in something useful like scientific research/general technological improvement (Galileo's discovering of deeper secrets of the heavens/improvement of navigation and cartography enabled by the scope)?