sale of gold coin

Reference: https://advisingfamilies.org/canada/information-portal/life-after-death/what-happens-to-assets/

"When someone dies, the government treats any property or items owned at the time of death as though it was sold on the day before which the person died. For example, if the deceased owned stock, it would be treated for income tax purposes as though the stock was sold on the day before the person died."

Thus, for the purposes of taxation, the value of the coins would be used.  If they were not appraised prior to being sold, the selling price should be used as their valuation.