@Separatrix Did a good job of explaining what short selling is. However, I can see a better option for short selling.
First of all, when your society crumbles, its currency will crash, and the worth of the currency will be zero. That means any cash you happen to have on hand will be only worth as much as the raw materials inside of it.
You need to get rid of all of your cash quickly. You could buy stuff, but it's cheaper to rent it. Picture you renting a horse for a couple coins. When society crashes, just leave the country. There will be no police or military to stop you, and the owner will be too busy finding food and staying alive to come after his horse. Buy or rent boats, animals, tools, and whatever useful supplies you can find. Make sure you get plenty of food too, because there might be a shortage. Don't buy land yet, because individual armies will try to take over the land for themselves, and without a government, anyone could commandeer your land. You also need to be able to move around at this point.
Travel to a budding civilization. If there isn't one yet, there will be soon. That new civilization will need all sorts of supplies to grow. By trading all of your stuff for land, you will infuse the civilization with the goods of a developed nation, speeding up technology and encouraging faster growth. When the small civilization becomes an empire, you will be a wealthy landholder. When that civilization starts to show the signs of crumble, you know what to do.