>So taking over a pre-existing site centered around illegal activity and simply seeing who uses it is about as far away from entrapment as you can get.//
Surely though if the site was shut down then the likelihood of the crimes being committed, that were recorded in those 13 days of operation, approaches zero? Would be interesting to see how many of these criminals when caught belong to more than one group.
A cop being undercover inside a criminal operation seems to me like a different category to a whole criminal operation being run by a department of police.
It seems close to entrapment; perhaps like going to an alcoholic rehab centre and offering alcohol around. Sure they could get it somewhere else but cutting off their supply should have been helping them reform already.
Surely that's the rationale behind targeting suppliers of illicit goods. If the argument is "the users will just get it elsewhere" then you should target the users and remove the demand.