Trading is a mechanic in Xenoblade Chronicles. It is possible with any named NPC, each having its own set of items offered for trade. To obtain an item, a material or collectable with an equal or higher sell value than the desired item must be offered. If the item offered by the player is of much higher value than the other, an Overtrade occurs and the player recieves a different item in addition to the regular item. The Overtrade items vary between different NPCs.
NPCs offer more items if the Area Affinity level is increased. They also offer different items depending on the area they are located in. A few NPCs like Désirée, Zain and Kurralth offer a different set of items in the same area depending on other things. Some items, such as the collectables from the Other Collectopaedia, can only be obtained through trading, and sometimes only if the NPC is in an area for a single quest objective.