RE:gold not a slide
Your question is justified!
But I assume that your thoughts are based on what you have learned from your parents. Means you can only sell what you own.
I think that this is still correct in regard to private persons.
It seems to be obvious that Banks, as well as the gray markets, are allowed by the governments to sell what they do not own!
They "stamp" the word "gold" on a piece of paper and sell it.
Because no authority goes behind it, it seems that this business model is accepted or even supported!
But you can be sure, if you and me try to act this way, we go to jail!