I'm surprised there are still markets that use BTC as the medium for illegal transactions.  This is about 5 years too late.  It's been about that long since Silk Road 1.0 and 2.0 were taken down.  Was under the impression that these criminals moved to more privacy centric coins for this type of activity (IE: Monero). 

BTC is poor to use for illegal activity since the chain is public.  The only thing private is the user's information. 

However as other commenters have pointed out, it's easy to perform chain analysis to figure out who owns what wallet and what the transactions are for.