A large number of games actually have a some what stable ECON, however, some of the newer titles coming out aren't so lucky.
This is due in part to the large number of flaws with said title. Id like to begin a discussion on how the developers of RIFT can ensure there wont be an unstable market crashes and price gouging that would cause inflation in the market.
A few things I personally have seen ruin good economy:
1. Expensive vendor sold crafting materials
2. Ultra rare drop non vendor craft drops needed for lower level crafting items of low to medium quality
3. Quest drop's that are able to be auctioned from mobs that are on strict spawn tables/times for important/progressive quests.
4. High drops rates on items for lower level players from higher level mobs (IE lvl50 death-cow has a 60% drop on a lvl 20 Super Blade)
These are a few things I could think of off the top of my head. I understand something will inevitably happen with the game just based on rule of thumb. Players will farm mats for brokering, players will farm elites for drops for brokering. And that's fine! but when the game is geared more towards the easy exploitation Vs. not, its easier for the economy to become unbalance and under-scaled.
I am also a huge believer in have a neutral point in which a cross faction broker is available to both sides, this assists in wavering any one faction standard on market pricing and helps set an over all lower market price on an other wise gouged item on one side.