In translating this expression, "dentist" may be a bit loose and refers to a broad category of "circus-like charlatans"

I'm not sure about small vocabularies, but French dentiste seems to cover most of the meaning of English dentist. Under the entry for mentir 'to lie' I did find: Mentir comme un arracheur de dents. An arracheur des dents would be a tooth-puller, and the saying seems to be a proverb of sorts. "Loc. proverbiale. [P. allus. à l'insincérité des arracheurs de dents affirmant que l'opération ne fera pas mal] Mentir comme un arracheur de dents." I did notice that English fair does not come into it.

Googling around makes me think that tooth drawer may be a better translation of arracheur des dents.

Ceci n'est pas un seing.