Merchant

Being a merchant is the proof of mastery of every job. You must be Grand Master in all jobs and bring the following items:
* 10 million crit
* 8 Sardonyx
* 12 Leaf of Bless
* 15 Diamonds

The benefits of being a merchant are the following:
* A merchant cape
* Able to whisper players from other nations
* Able to use the market board for selling items
* Cheap access to other nations
* Able to upgrade to a Saint Merchant which allows you to combine two different rares together

From the help built into the game client.