I can't say I abide by it 100% of the time, but I do sort of like The Blumert Test he mentioned: check the coin you are considering in a 10-year-old copy of A Guide Book Of United States Coins. If there is no evidence that the coin was rare 10 years earlier, tell the guy you can’t use his coin.
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Thanks for the comments, Fred & Lermish !
I can't say I abide by it 100% of the time, but I do sort of like The Blumert Test he mentioned: check the coin you are considering in a 10-year-old copy of A Guide Book Of United States Coins. If there is no evidence that the coin was rare 10 years earlier, tell the guy you can’t use his coin.