Say I have some silver half dollars, but no receipt of purchase. Can I claim face value (50 cents) as the basis should I sell, or is the basis nothing?
I don't think zero would ever be ones basis for this type of situation. The face value is as safe harbor as you'll ever find. While you need to pay on this gain, like kids selling baseball cards, I don't know that average folk are actually claiming such small transactions.
The I.R.S. allows retroactive creation of cost basis for coins and stamps if you don't have receipts. Here's a link from an article in the Wall Street Journal that describes how to do it. Note that this was dated December 2008, so you'd need to confirm if it is still allowed by the IRS now!
It seems to work like this: You go to a coin dealer and tell them when you think you acquired your coins, item by item or lot by lot. Start with the coin(s) you think are most valuable. In your case, you have all silver half-dollars. Do some have greater value than others, based on
That is why you need a coin dealer to help you. You and the coin dealer will start with your best effort recall of when you acquired the coins. The dealer will give an estimate based on that date as to what the fair market value for the coin was, and record that as your cost basis. You'll do this for all your coins, and submit the results to the I.R.S. along with the amount you received for selling your collection.
Needless to say, you should get the coin dealer's retroactive cost basis estimate done before selling the collection! And you should probably not sell your collection to that same broker either!
As others have said, you will be paying capital gains taxes on the difference between the acquired value of the coins and the sale value. The minimum basis will never be $0.00 for the acquisition value, which was part of the question. That's because the I.R.S. will always use the value as a unit of currency as a minimum assessment of worth.
The minimum value would be the face value. That is the non-numismatic value of the coin. In your case, $0.50 per coin. So valuing all the coins at non-numismatic value maximizes the amount of taxes you would pay upon sale of your collection.
Be aware of this, though! The coin dealer won't do a retroactive cost basis estimation for free. So you need to consider that expense to decide whether it is worth it or not.