Need – You should choose need option if you can use the item and if it provides better stats than your current armor. Companions are important for every player, but the player’s character always have priority over any companion. Needing for a companion, without asking the group first, is usually an instant kick from the group.
Greed – You should choose greed option if you need an item for a companion, alt or a friend, or if you just want to sell it. The same rule stands for crafting materials, if you cannot use it choose greed option.