Remember that the goal of the Merchant Guild is to control at least 20% of the economy of Rune-Midgarts. Likewise, they also have the Vending skill, which is very useful when trying to sell items and equipment to other players. Merchants have the Discount and Overcharge skills, which lower and increase NPC price values by as much as 24%, respectively.
With an array of skills tailored toward zeny production, the Merchant class is essential for every Ragnarok account.
When it comes to money-making, no class specializes in it better than the Merchant class.