Learn Spanish (Latin America): Francamente, el agente no se fía de nadie. - Frankly, the agent does not trust anyone.

This is an example of a lexical shift. The base verb fiar usually means to sell on credit or to vouch for someone. When you add the reflexive pronoun to make it the pronominal verb fiarse, the meaning changes entirely to "to trust."

Why is there a de after se fía?

The pronominal verb fiarse always pairs with the preposition de to introduce the person or thing being trusted. You can translate the whole chunk fiarse de as "to trust" or "to rely on."

Why does the sentence use both no and nadie? Isn't that a double negative?

Yes, and Spanish requires it here! When a negative word like nadie (nobody or anyone) comes after the verb, you must put a no before the verb. If nadie came before the verb, you would drop the no, as in: Nadie se fía del agente.

Why is there an accent mark on fía?

The accent mark breaks the diphthong between the weak vowel "i" and the strong vowel "a". This shows that the stress falls heavily on the "i", splitting the word into two distinct syllables: fí-a.