I know this is short thread but It's come to the conclusion without official notice but logically thinking, the Key Item will be taken from inventory when the Relic Weapon quest is complete because It is still allowing me to purchase another "On the Properties of Beastmen" item. I know I can't buy another one atm because I still currently hold one but it hasn't hidden itself from me doing so as the Chocobo Barding KeyItems do.