Not just commodities can be bought and sold, money can be bought and sold too. That's why there's a giant market of currency exchange. You're creating another false dichotomy that money and commodities are always entirely separate things (sometimes they are, but not always). The other false dichotomy is that money and credit are separate things. In reality, its blended together on a spectrum: pure credit/currency on one end, and commodities with no exchange-value on the other.
Still, gold also has a use value independently of being money, as does cattle, silver, and salt. They are also useful "regardless of whether or not I trust you," and even so they can be - as once were - money.
Its having a future exchange-value based on trust which makes something a currency. Not "future exchange-value based on use", which is a total misuse of the term exchange-value. future exchange-value (the gold) is a separate thing from future use-value (the knife).
But we do agree, having a use-value does not preclude something from also functioning as money. Commodities can have a dual-function as both currency and as commodity (again, in the real world, its a complex blend). The point is that, as a currency, its exchange-value is derived separately from its use-value. The exchange-value comes from the network of trust among people who will accept it as payment, even if have absolutely no intention of using it themselves (just an intention to exchange it again in the future).
So its metallic utilities would make it useless, then credit, hence money?
No, its ceremonially-precious quality would give it exchange-value (derived from the trust and agreement that it is precious). That is entirely separate from its utility as a cutting tool (its use-value).
Okay, credit/IOUs have value based on trust, that should be obvious (an untrusted IOU is worthless). Money/currency have value based on exchange. And that exchange-value is always in flux, clearly, the price of gold and bitcoin are constantly changing. Changing based on what? Based on the trust that it can be taken to the market and exchanged at x price in the future, in other words, that it can serve as a "good-as-gold" "buyer-owes-u" x in the future. If that trust collapses, the price collapses.
Physical gold is just a token which measures the credit backing by the gold market. So instead of an "i-owe-u" its a "gold-market-owes-u". And obviously a "gold-market-owes-u" is only as valuable as the collective trust in the gold market.