Wiki - Merchant Coins
Every character begins with a Merchants Pouch in their backpack. Double clicking on the pouch for the first time claims the bag and allows the collection of Merchant Coins.
While a character has a Merchant Pouch in their main pack, Merchant Coins will be collected for harvesting any resources, crafting any items of sufficient material, and for filling BoD's. The higher the resource gathered or consumed the more coins will be earned and carrying the pouch in a bag inside the main pack won't work.
By double clicking the pouch a character will open the gump to the right. From there players can see how many Merchant Coins they have saved. They can be extracted as a check or as coins, and may be sold for 50 Gold each to a Merchant Guildmaster which can be found at most banks.