"Ви продавець?"

Translation: Are you a sales assistant?

[BogdanBolbot]

«Are you a sales» considered wrong.

[Tokhtuevs]

It would have been "seller" instead of "sales" then. But no native speaker would say "Are you a seller?" either.
"Are you a sales assistant?" sounds a bit clunky too in English, normally you would simply ask "Do you work here?" if you want some help in the store.