The merchant will have a slightly different appearance depending on the location, but will be among a group of Devotee's of Flame that are casting spells to set off a fireworks effect. You can tell you are approaching when you hear the fireworks sounds.
The flame-themed enemies (all elementals) near the Devotee's of Flame celebrations drop the Coin of Ember currency needed to buy most of the items from the Scorched Sky event.
The Commonlands at Dog Trapper Lake, on top of the giant rock. A tiny ramp at ( 14, -44, -733 ) /waypoint 14, -44, -733 provides access. Those with flying mounts can go right to ( 11, -13, -750 ) /waypoint 11, -13, -750