To my knowledge, it is illegal to sell exploit kits to actors that you know are going to use the kits to commit crimes (e.g. if someone sends you an email saying "I'm looking for an exploit kit so that I can attack company X and steal their IP", you cannot legally sell to them). It is otherwise legal to sell, rent, or give away exploits to the general public or to resellers like Zerodium as long as they are not marketed as criminal tools.