New money can literally be printed by a player on an everyday basis. In a world in which productivity is measured by killing  monsters, looting some copper, and respawning a harvest node dropping a chest, the money supply is unlimited.  There is no way anyone can gain a monopoly on any of these activities.

If a particular Guild was the only one with access to particular items, price fixing would be a problem. It isn't because this isn't possible.  Masters drop for everyone, as do solo mobs.

at the high end are different. Not everyone is in a position to acquire them. However, these are rare and unlikely to create market chaos - especially when there an abundance  of drops, crafted items, and quest rewards available.

As the supply of masters and rare loot increases over time, prices will fall.  It's possible that some player with sufficient means could use certain tactics to dominate the market for a time, but such an effort could not be sustained for any length of time because of all the new resources constantly coming into the virtual world.

An MMO economy isn't like the real hyper-capitalist corporate world in which a few powerful individuals suck up all the wealth, leaving most others with little chance to acquire any. Even the most humbly equipped character in EQ2 has the chance to find a master or loot something valuable every day with just a little effort.

Maybe that's why we love these games so much...

Via EQ II Forums