All I can say is that if they are used, they are very uncommon. I never met anyone with those names.
My sister says she read about a girl who, when still nameless, was in a car accident and crashed on some boulders. Her parents died, and the orphanage decided to call her Sela after the cause of her parents' death. Depressing, if you ask me. Not that anyone would like to be called Sela now anyway - there was a rapist on the news not long ago with Sela as a surname.
Shelach - I've never heard of used as a name.
Both are as strange to me as calling someone Missile or Boulder (because a sela's more a boulder than a rock). They just aren't names to me, though some creative parents might use them (but there aren't creative parents here like those that are in America by the sound of it).