It means that for black markets transactions, which are made illegal by the state and thus impossible to discuss in the open, more peaceful ways of settlement (e.g. arbitration) are not available.

This implies that instead of a doctrine of proportionate response, you end up with a binary system where you either get away with petty theft until you don't, because the guy you steal from has had enough and wants to end you.

In concretu: If Silk Road was legal, the owner would have reacted with a lawsuit in the court system. It's not, so he had to resort to violence.