But none of them were able to make a move despite knowing Anthony's current location.
Why?
Anthony's airship was simply too fast for them to follow endlessly. They needed him to land or stop before they pounced upon him. Be it at the Null Sect, nobody would have any trouble infiltrating such a Sect.
As for the people who cashed out on this? They were merchants, obviously. Using the sky-high morale people possessed to kill Anthony, many merchants increased the price of assassination-type equipment and the likes, or anything that could increase the chances of killing Anthony.
After all, anyone who killed him would gain immense prestige, aside from the already promised payment from the bounty.
