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.
you obviously forgot how much the government made from selling of his bitcoins.