Full Idea
Being sincere [in Sartre] has nothing to do with authenticity because, in committing ourselves to a particular identity, we strip away the possibility of transcendence by reducing ourselves to a thing.
Gist of Idea
Sincerity is not authenticity, because it only commits to one particular identity
Source
report of Jean-Paul Sartre (Being and Nothingness [1943]) by Kevin Aho - Existentialism: an introduction 6 'Bad'
Book Reference
Aho,Kevin: 'Existentialism: an introduction' [Polity 2014], p.101
A Reaction
I take this to mean that sincerity says genuinely what role you are playing (such as a waiter), but authenticity is recognition that you don't have to play that role. I think.