If it is a non criminal situation, you can generally use traditional methods to establish trust, such as using courts to sue people who rip you off, or using escrow agents, etc.

The world has had thousands of years to come up with solutions to trust problems during transactions. They are solutions that work. They work for criminals too, but its a lot harder to use them, especially over the internet, because everyone providing the services goes to jail if they are transacting with criminals.