Guns (well, swords and truncheons before that) and tight cliques of trusted human gangs, mostly. And I think I'd argue the dark net markets work the same way: holding reputation as currency, they provide an incentive for the users not to cheat, but if they cheat, there's no recourse except "Well, guess I'd better find another market." And that's if we discount the few but notable instances of them using actual violence to enforce their norms... Ulbricht of Silk Road got arrested for attempting to put an actual bounty on someone's head.
The blockchain system provides a simple way to very publicly and reliably declare the exchange of meaningful numbers between two pseudonymous identities. Everything else about the market that blockchain exists in is the same grabasstical process that humans have used for exchange of resources throughout all of history.
The key thing they do is include a third party in the transaction, usually the market itself, who mediates disputes along with a reputation system for sellers. They mostly do it in a centralized way, where the market holds your payment until the buyer confirms or the seller can prove they held up their end. This is a problem because the market can and often does exit scam, so some do this in a decentralized way with 2 of 3 multisig transactions, where if the buyer and seller have no dispute, they can release the money, but if they do, a third party arbitrator can come in and mediate.
Also, higher layer payment protocols like the lightning network effectively clear instantly and trustlessly, with no need to wait for any block to be mined, and are great for minor value transactions.
Finally, replace bitcoin with gold in your question, and ask why that worked for thousands of years.