Here is a list of items unlocked by the "Privileged Customer" perk for each merchant type:

Blacksmiths: with "Daedric item alternative" MCM option toggled off: one piece of Daedric weapon/armor.
Blacksmiths: with "Daedric item alternative" MCM option toggled on: a couple of glass/ebony weapons/armors with highest level enchantment.
Fletcher/Drunken Huntsman: with "Daedric item alternative" MCM option toggled on: one Daedric/arrows/dagger.
Fletcher/Drunken Huntsman: with "Daedric item alternative" MCM option toggled off: a couple of glass/ebony bows/daggers with highest level enchantment.

Alchemists: two ultimate potions and a couple of rare ingredients.

Mages: one high level staff and scroll, empty/filled black/grand soul gems, a master robe and two expert level spell book.

Tailor: Jarl clothing.

General Goods: a collection of skill books.