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There are always opportunities to sell gold and silver coins so I do not think that would be an issue. Whether or not the coins will be worth more money at the time of sale vs. time of initial purchase is the question. Some coins do appreciate in value while others do not, and you would receive the face value of the coins.
Please keep in mind that if you sell precious metals for more than what you paid for them, chances are pretty good that a tax liability will exist. You will pay capital gains tax on the difference between the basis (the purchase price when they were bought) and the selling price. The maximum capital gains tax rate on collectibles is 28%. However, in the case of an inheritance, the basis of the metals is equal to the market value of the metals on the date of the death of the person that left the metals to you. If you were to sell the coins very soon after you inherit, there will probably not be any capital gains tax.
The following link contains excellent information which you will find helpful and which you may want to show your dad:
Please let me know if you require further information or clarification.
Thank you and best regards,