How is something like this still allowed to continue?

[Seraphor]

Why don't SE get a GM disguised as a player into each of these sellers to see if they sell it by gil or money? Is it that dificult to join their discord and catch them in the act?

As has already been said, they don't take external data into account.
Once a GM joins a discord, they can't be absolutely certain it's connected to the account advertising in game.
It might seem like common sense, but that's life.