If I go get change for $10,000 in $100 bills and ask for all $1's do I realize 10,000 dollars in capital gains because those are "other people's dollars"

but they're dollar bills, the currency we pay taxes in, they're always worth $10k no matter what happens to their actual intrinsic value ... a better example is:

I buy 10oz of gold for $1000 (this was a while ago), this week I exchange my gold for a different 10oz of gold (less a small commission to the exchange) currently worth $10,000 - is that a taxable transaction? I suspect yes