Gold/currency: Typically, the most common form of payment. Used for everything, most merchants will allow multiple forms of payment however, this is the most common and well known one.
Services/quests: Another form of payment. Commonly used by potion brewers Bakers, weapon smiths, armor makers and etc. This is commonly them requesting you to go on some quest to get an item for them. So that they can continue to produce their stock. After you complete the quest, you may obtain the item you requested from them.
Servitude/pet service: The most uncommon form of payment. You agree to become someone's servant/maid/pet for a while and in return you gain their item/service they do provide. Normally it's just a few hours based on the rarity of the item.