Like Peachey said, it is merely an inverted sentence structure. .'you' is the subject and follows the verb 'are', of course a derivative of 'to be'. Because of the absence other grammatical elements that could be confused as the subject of the clause, the meaning is clear ("you are seldom believed"). Generally the verb is placed second with the subject after. Because the English language is often vague with pronoun cases, you won't often find sentences such as "seldom believe they you" where 'you' might initially be confused for the subject. Sentence inversions rarely stand alone, and are considered somewhat archaic, but when used with dependent clauses it is a good way to shift emphasis (in this case emphasizing 'seldom' while de-emphasizing the subject 'you' ) and just sound classy. It's almost never used in casual speech.

First thing: "a" = singular, but "treasures" is plural. "A treasures" is not possible in English. The grammar you are asking about is called "inversion", and is advanced-level grammar.