If someone is buying weed on a darknet site, would you feel morally justified in stealing from them?
A guy selling credit card numbers, given the opportunity he'd have nothing left.
No, but I'd guess that GP feels morally justified in buying weed from him or selling weed to him.
If the weed-buyer isn't playing the theft game, why do you believe that the GP would play the theft game?
If your actions have blowback on morally innocent people (ie. Technically a law was broken but it was a crime without a direct victim), inadvertently or not, you are not acting morally.