The Witcher 2: Assassins of Kings[]
Nekker eyes are used as crafting components, alchemy ingredients and also as quest items. Two sets of eyes in particular are listed as quest items and as such valued at 0 
and a weight of 0. They are also used to craft amethyst dust.
Associated quest[]
The Witcher 3: Wild Hunt[]
Nekker eye is an alchemy ingredient that can be dismantled into monster eye. It is needed to craft Enhanced Hanged Man's Venom.
It is a random drop from nekkers and their monster nests.