For reference: Cloaks in the game:http://www.exiledkingdoms.com/wiki/index.php?title=Back
In general, sell value goes up with item quality. But just because an item has a higher (or lower) re-sell value doesn't mean it will have more (or less) usefulness to your character.
For cloaks, the Nivarian Cloak offers limited benefits (+8 hp. +12 Death, +15 Poison), but can be gotten early in the game and is one of the few items that offer poison protection and the death protection stacks well with early game death ward rings. So early game it's not too bad. But by mid-game, and end-game, you'll have a lot more Death and Poison protection items. Given the game mechanics on how elemental damage works, you might not see a difference in damage based on a mere 12 / 15 increase.
By comparison, the Iron Will Mantle offers +13 hp and +25 Spirit Resistance, with the spirit resistance being the attractive aspect.
Before getting stun immunity skills or items, getting stunned can be deadly. And even after getting stun immunity items or skills, having a high spirit resistance still helps with enemies that can paralyze. With spirit resistance items being rather limited, the cloak is rather nifty and maintains it's usefulness though the entire game.
Keep in mind, the usefulness of any equipment needs to take into consideration the class and build of the character as well as the dungeon at hand. (No point in having Death or Poison resistance in a fire cave, etc.)