Hello,
To clarify, you want to sell the entire ninja and then be unable to recruit them? Or do you want to sell individual fragments? Or something else?
Please let me know, thanks!

Alright, I will submit this suggestion for you, but please be aware that it is very unlikely to be implemented, even at a small fraction of the ninjas' base value. If the devs wanted players to get coupons from every event, coupons would be an option in every event, and the fragment refining system would not exist.