Whenever you talk to the necromancer (you may need to go into the creature creation screen), any new body parts in your inventory get added to his list (any remaining after that are duplicates, and can be sold). The same is true of alchemy potions and enchantment formula, with the alchemist and enchanter.
Even if you accidentally sold the parts, you will be able to find better by the time you finish the necromancer's quest and are able to summon one. Later in the game you will have access to a merchant that sells body parts, as well. Other than random loot, there are also a couple chests and quest rewards that give body parts.
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