I think the reason it seemed Gold Mario had little trade potential was because a lot of demand for looks to be from the resellers themselves or those who wanna use it to trade for what they do want. But in terms of value through rarity it still has that.
But if you think of utility, a Gold Mario is really no different than a regular Mario I'm believe.
Yeah basically. All the collectors and traders were able to get one, so they don't need it. But the general population that is not in the know about amiibo but still buy them, didn't manage to get one so the demand is still there for them as it is impossible to buy in stores anywhere.