Merchants are NPCs that will create Trade Offers, though with a harsh ratio of 1:10 meaning they'll trade 1 of something for 10 of something else. You do not have to pay a Forge Point when trading with a Merchant. It is not recommended to trade with them, it is at your disadvantage.
Merchants will only place offers of the same tier, for example; Rope for Glass and Marble for Lumber or Wine and etc. You can find out more about the Tiers by reading Goods. Merchants are considered as the last resort if you do not have access to Diamonds.