What are Giant Skull Necklaces for?
 

Does anyone know which repeatable quest they're for? And where?

I think its quest in Belcar Plateau. Either your lvl is too low for the quest req or you clear a boss killing quest which makes the rep quest disappear or its just for a normal quest which the the drop is normal item (not quest item drop).

I see. Should I just sell it at the auction house then?

Yes I think its not for a rep quest (the NPC for it is at observatory if I remeber right) and the quest needs like 10 or so and they are pretty easy to get so might aswell sell them if you need space.