In cities with police forces. Purchases at street-markets don't devolve into robbery because police forces disincentivize that. The municipality taxes you for that service, and so is effectively a middleman and rent extractor for these transactions.
Also, in cases where you've decided to trade using cash rather than to barter, you're being protected from your counterparty paying with counterfeit currency, by the state coming down rather harshly on counterfeiters, keeping counterfeit currency out of the market. That's a service they provide in exchange for state-level taxes. So, another middleman and rent extractor in your transaction.
> one off websites
When this works, it's because the payment processor (Stripe, PayPal) is acting as the middleman and rent-extractor.
Even without a payment processor, you can usually seek redress from your credit card company (another middleman and rent-extractor.)
Also, a search engine probably acted as a matchmaking agent to lead you to reputable services (and so is also acting as a rent-extractor in a round-about way, with sponsored ads and such.)
Without these parties in play, the Internet is not a safe place to transact. 99.99% of credit card input forms on the internet (by volume) are those of phishing domains. That's what a non-iterated prisoner's dilemma looks like.