With two parties, whatever exchange rate they prefer. They don't need to worry about the total supply but if they're curious, sure. Supply and demand work as usual: subjective value on the margins. If there are large deposits somewhere, they pale in comparison to total mined supply. Yes, you do need to guard it, kind of like your keys. The safely exchange question applies to any honest money, but if you already know a that second party who will exchange, you're OK. Authenticity is a legitimate concern.