I see post about this every week. As I understand it, there's a vendor, on weekends, who will sell helms and shoulders that are normally only available from boss drops. Do I have that right? And this guy appears in Cyrodiil? Where do I find him? What are the costs? The type of item is random each week right? Does he sell a helm and a shoulder piece or just one or the other?
the vendor appears at one of the 2 home bases in each of the factions, she sells 1 of the monster set pieces (either a head or shoulder) and 2 PVP set pieces. They gear available changes from week to week. They are available for either gold or AP. If I remember right (haven't checked her out in 5-6 weeks) the monster set pieces are available for 90k gold or 200k AP and the PVP set pieces are for 60k gold or 100k AP.
Either a helm or shoulder. All weights, in either infused (for 100k gold) or impen (for 200k AP). There will also be some random PvP set pieces, sometimes jewelry.
Remember, never waste your AP or gold on any of the PVP pieces. You can always find them cheaper in a trader. Even when it was something desirible like Vicious Death rings, I got it for 30% of the gold price out of a trader because ZOS does not understand the appropriate Gold/AP ratio.
As for the monster set, you need choose your currency based on the trait you want. Very frustrating that PVEers get infused with their hard earned gold, which is generally a lousy PVE trait, and PVPers get Impen, which is arguably the best PVP trait. This is why I havent checked the trader in months. They have nothing I want.