Reputation. People don't necessarily just go out and buy $5000 worth of goods from an unknown seller. They start small, with a value they're willing to lose. Say $20. Over time, the seller consistently delivers the goods requested, and so their trust in the seller increases.

You can imagine that with a consistent system, this reputation can be transmitted/communicated to new parties. Think EBay for a clear example. Perhaps not the best of analogies, as EBay has money back guarantees/insurance, etc. But as long as drugs remain in the dark underbelly of government-hate, don't expect much protection from conventional institutions.

I believe the system is sort of like Ebay, do that too many times and people will leave negative reviews and future buyers will not do business with you.