You can't arrest or put people in jail when no crime has been committed or even before one has. They don't have some sort of Minority Report Pre-Crime Internet Unit. They'll just have to learn to deal with it when it comes and go after the individual dealers which is what will happen.
It will be interesting to see how tracking down individual drug dealers will be effective with a decentralized marketplace with strong privacy protections built in. It took the combined efforts of multiple departments and millions to catch Ross Ulbrict. They may find that they have a limited budget and need to focus on investigating murders for hire and child porn instead of victimless crimes like drug dealing.
The idea is to make it unaffordable to fund the war on drugs with asset forfeiture alone. Drugs will become legal and regulated because it is obsolete and too expensive to attempt to criminalize it. They will be forced to accelerate decriminalization and legalization which is a great step in both reducing addicts, preventing disease, and liberating society.
I expect large drug kingpins to be caught through enough effort and investigation on openbazaar, but many small dealers will slip by in a thriving market of both legal and illegal goods and services.