authorities cannot post illegal ads
Where did you get that idea?
One of the worst examples of this was done by the U.S. Postal Police who, until the rising of the internet, ran KP ads in the back of most major men's magazines.
There was a time that the largest distributor of KP in the US was the US Postal Police, accounting for well over half the KP distributed in the US. They would actually mail KP to the seekers and bust them when they went to pick it up.
This was the police obviously not only planting ads suggesting illegal items for sale, but distributing them.
Not something I ever thought well of. It was as if the NYPD/NTF began running the biggest heroin street distribution biz in the city in order to catch street level users, or becoming the biggest pimp to bust johns. Truly bad karma.
I am all for getting rid of child molesting scum, but not by becoming a child pornographer in the process{shudder}.
There are recent examples of international, multijurisdictional [including the US via ICE], taking over of KP sites already existing outside the US, and busted [far better idea], and using their already existing street cred to capture users of this stuff in the US and infiltrate distribution networks world-wide.
During the time of their operation they would give the appearance of being an illegal operation, including running CL, Usenet and other ads in places where those expecting to see that kind of shit would look for them.
So to tie it up, while many in law enforcement aren't crazy about the idea of becoming the thing you are trying to kill in order to kill it, it is an effective tool and IS USED. That includes acting like the thing would act, which means running ads to maintain the appearance that you are that thing if that is what is expected.