> If you want to buy a car and the seller agrees to accept 50 gold coins instead of $100,000 cash, that is perfectly legal.

You're free to barter in general. 50 gold coins, though, would probably be illegal even though 50 marble statues is fine.

https://www.law.cornell.edu/uscode/text/18/486

Using gold (or any metal) as currency ["current money"] is specifically illegal if the metal is coined.

You'd need to establish that it never crossed the seller's mind that he might later exchange those coins for something else. As an isolated incident, you'll have a fairly strong defense. If there's been another transaction in gold coins in your area recently enough that either of you might have known about it, you won't.