Gratified merchants spawn in those locations where distressed merchants have been successfully rescued from the monsters that surround them. Gratified merchants appear in semi-random locations (again, only after you've rescued them) in the following zones:
For more information on spawning gratified merchants, read the information about their placeholders, distressed merchants. Once you have spawned them, gratified merchants will either buy items from you or sell items to you, but never both. When they buy items from you they will always pay you the highest prices possible for those items. When they sell items to you, they sell a variety of things appropriate to players in that tier such as food and drink, various kinds of ammunition (e.g., arrows and thrown weapons), and certain quest books that can only be bought from them (for example, the book that offers the quest Fire and Ice Creature Cataloging).