There are some unsellable Items in the Game. Some of them are important for Quests. I dont know if this is the reason theyre not sellable. But some of them are NOT important for Quests and for all of them there is another way to solve the Quest, so there is no need to make it unsellable.
Items:
Migo's Ring = From Migo in Fort Joy.
The Shadow Prince's Signet = You get it from the Shadow Prince when you kill him on the Nameless Isle (not quest relevant)
Gloves of Acquisition = You get it from Dashing June on Reaper's Coast. (not quest relevant)
Radeka's Purging Wand = From Radeka after you kill her in Fort Joy.
Lone Wolf's Mark = From Baran Levere on Reaper's Coast in the Tavern. (Afaik not quest relevant)
Captain's Compass Amulet = Can be found on the sunken Ship Peacemaker on Reaper's Coast. (not quest relevant)
Mordus' Amulet = From Mordus on Reaper's Coast.
Hannag's Ring = Can be found in a bag near Gareths House on Reaper's Coast.
Jonathan's Ring = Youll get it after killin Jonathan on Reaper's Coast.
Livewood Shield = From the Ghosttree in abandoned sawmill (Not quest relevant)
Black Ring Blade = From Owin Anchoret in the Black Pits in the burning House which is attacked by Magisters. (Not quest relevant)
Brocken Source Collar Amulet = You can steal it from Sir Gareth (not quest relevant)
Fran the Bicorne Hat = You get it from Duna in the Duna Temple on The Nameless Isle when Beast is in your Party (not quest relevant)
Spear of Foes = You get it from the Elves on Reaper's Coast near the abandoned sawmill after you free saheila (not quest relevant)
The Groom's Gift = Can be found in the grooms house in Arx. (not quest relevant)
Noble Hat = Can be found in Arx.
(So what? If youre not noble you don't get tea from Lady Kemm)
Linder Kemms Wedding Ring = Can be stolen from Linder Kemm.
(So what? If you sell it you have to fight or sneak the Statues)
There are many more i have found in other playthroughs but those are the things i found in my actual playthrough.
Latest Version of Definitive Edition
Localisation German
No Mods used
