First of all, it is not a coin for buying things.
But since the phrases are spoken by more than just Faceless Men, they seem to have taken on more universal meanings.
Finally, Arya met a man called Ternesio Terys asking him for passage to the Wall.
" The last thing you want to do if you're in the capital city of Kings Landing is get caught walking around in your smallclothes.
Daenerys, who's made it to Astapor, strikes a deal with the slave trader Krhaznys mo Nakloz.
He gives her a coin with a "Valar Morghulis" inscription and tells her to say the words to anyone in Braavos if she does want to find him later.