Since there are no Merchant Classes inside Ragnarok Mobile, the use of the Auction System is quite profound. In the Auction System you'll find various items people are selling ranging from equipment, gears, crafting materials, usables, cards, etc. Here is the basic interface:
Where do I find the Auction System?
You can basically find the Auction NPC in every town. Look for a guy in a suit with black spiky hair and glasses. In the image below, I am currently in Geffen and that is the Auction NPC.
Selling Items in the Auction System
When you are selling items in the Auction System, you just don't simply put in any zeny amount you want to sell your item at... the selling price is already calculated based on the average prices currently in the system. I believe it's based on the rarity of the item, difficulty of creation, drop rate, etc.
Cards in the Auction
Just a simple interface with the cards in the auction system. If you don't remember what the categories are, you can always look at the small little icons beside the monster's image. That will indicate where the card will compound to.
Edited by Mystery