MANA's price has a tendency to decrease because
1. Each new user get lot of free MANA
2. User can get 260 MANA from daily quests
3. User can buy any amount of MANA using real money

This uncontrolled increase of total MANA cause inflation

BUY If you have unused MANA and want to protect it from inflation
SELL if you want to get back your MANA and use in other markets
SHORT if you expect that lot of traders will sell their stock to get MANA and use in other markets(this might happen if more attractive markets appear, like presidential election)

If many users follow above instructions, the stock price in this market will be correlated with total amount of MANA. As a result, the price of this stock should consistently rise over the long term because there is a limited number of shares available. This makes it similar to gold, which is often viewed as a hedge against inflation.