That seems like it wouldn't be cost effective for them if there is even a small amount of anonymity between the buyers and cultivators.

I mean, cartels have been pumped up to pretty ridiculous heights by the war on drugs but it's not at the point where it would be realistic to have them say "here is a list of a few hundred usernames from SilkRoad spread out all over North America. Find them and murder them all."

Even considering that their MO is more along the lines of picking out the ones they can find and making a gruesome spectacle to scare the rest it still seems unrealistic, although so do a lot of things happening with drugs cartels these days to be fair.