Dark Souls Remastered | Domhnall of Zena in the Depths full dialogue

In my opinion, the highlights of what he sells are the crystal items, including:

Crystal Straight Sword (4000 souls)
Crystal Greatsword (6000 souls)
Crystal Shield (4000 souls)

Crystal weapons and shields are unusually powerful, but cannot be repaired with the exception of when they're upgraded. That means whenever they break after reaching full upgrade, it's no longer good for anything and you can discard it.

You may find some potential useful gear upgrades from him.