Most commonly, an NPC will request from the players in the area to collect certain items within a clearly marked geographical location on the map, either by interacting with an object or looting from defeated foes, and bring them back to them. Once the NPC has collected enough items, the event will generally complete while the NPC may run away and create a consumable or other goods offered for sale. The event merchants with their items for sale are only available for a few minutes after the event ends.
Included within this category are Legendary collectible items, usually provided after completing a specific events or action. These items have no intrinsic value outside of the collection itself, cannot be sold or salvaged, and are account-based, which means that as long as the legendary collection is active on the account, any character can obtain the collectible. To avoid clutter, these items can safely be destroyed after you first ensure they have been successfully placed within the collection. If any collectible was displaced or accidentaly deleted, they can be recovered and purchased again from Grandmaster Craftsman Hobbs in Lion's Arch.
With some noted exceptions, most event items have rarely any value outside the event. They can be sold for a small amount of coin or destroyed without consequences.