How is zakat to be paid on stored gold coins that are not intended for trade, knowing that they are sold in the market at a higher price than pure gold due to their antique value?

If the gold coins are not for trade, then what matters for their Zakat is their weight, not their value. Zakat is to be paid at a rate of one-quarter of one-tenth (2.5%) of them, or that weight in gold, or its equivalent value in circulating currencies.