In a lot of real-world interactions, the trust is rooted in an established direct relationship. When I contact my credit card company, I do so by calling the phone number printed on my credit card. When I mail them a check, it goes to an address that I am familiar with from when I opened the account. Also, when they receive the check, they actually request an electronic funds transfer from my bank, and my bank and my credit card company have identified each other from experience, from looking at the identifiers on my check, and by their government charters.
In none of these cases do the parties use an arbitrary trusted authority. Either they have shared information that they use to identify each other as being in an established relationship, or they have a specific body, with whom they have a direct relationship, that they use to introduce them to each other.