You can’t sell Gully’s starter (Aramus’ Gloves) or Garrison’s starter (Artifact Blade). Not sure if they can be upgraded later on in the game (most likely not) but I believe the devs decided to make them unsellable simply because they’re very important to the lore comics-wise, even if they end up not serving any purpose in the game.
Also, you won’t be able to sell the blade you pick up at the end of clearing the Iron Outpost, just so you don’t need to wonder later on! (I believe it’s called Red Rusty Blade or something to that effect)