I looked around online but couldn't find a good guide detailing where the vendors who took Sequins (Solomon Island, Valley of the Sun God, and Transylvania) are and what they offer (specifically what QL gear they have). Initially I was content to discover them on my own as I progressed, but then I was irritated to discover that I would have been better off saving my Solomon Island sequins for the vendor in the Blue Mountain, rather than spending them in the Savage Coast.
The Sequin Vendor for Kingsmouth Town is just north of the police station; the one in the Scarred Coast is next to the fish and tackle store, and the one for Blue Mountain is in the main building of the Wabanaki Reservation.
The vendor for the Scorched Desert is in Al-Merayah, next to the Council of Venice representative for the area.
In the Besieged Farmlands I think you can find the vendor in the inn, and I'm certain that the vendor in the Shadowed Forest can be found in the Draculesti camp.
They're marked with a blue-on-white pile of coins on the map, and in exchange for sequins of the appropriate area they sell blue grade weapons, talismans and glyphs of a quality level appropriate to the area.
Kingsmouth Town (153, 375) don't buy stuff here!
Savage Coast (128, 854)
Blue Mountain (245, 643)
Valley of the Sun God
Scorched Desert (601, 436)
City of the Sun God (400, 52)
Besieged Farmlands (1007, 1035)
Shadowy Forest (853, 872)
Carpathian Fangs (710, 681)