With the cost of Nirncrux being so high and availability limited, I have some suggestions for requesting nirnhorn items for trait research.
A) Make request as normal (maple, jute, iron item) but state in buyers notes you will provide the nirncrux.
B) Request a maxed out item, level 14, rare motif trait, 8 piece researched set, then state in buyers notes "for trait research" and a low lvl item will be provided.
Following either of these suggestions would reward the Crafter appropriately and increase the probability of your requested item being made.