Group Event Quest Merchants (not official name) are those guys that appears after a group event. They will sell stuff that are not normally available. They usually goes something like this:
1) NPC1 asks player to do a quest, and starts a quest chain.
2) NPC1 runs around to location A, B and C during the quest.
3) At the completion of the quest, the NPC1 will run from location C back to A.
4) NPC1 change into a merchant.
I personally find part "3" highly annoying. To buy stuff from the said merchant, the player have to run from C back to A for no reason. And often times the run from C to A takes 5 minutes or more. NPC1 should just stand and change into a merchant right at location C, so the players can buy the item right there.