A question I get periodically is: is this legal? Absolutely, yes.
The most common misconception is that any defacement of US money is illegal. Sine I don’t deface any of my coins, the issue is moot, but I’ll address it regardless. Defacement is only illegal if it’s done for the purposes of fraud. If all defacement was illegal, you wouldn’t be able to have those machines at tourist sites (including national parks) which flatten and imprint pennies with designs related to the location.
As an aside: It is also illegal to melt down modern American pennies and nickels. This is because the coins’ metal is worth more than their face value. Which is dumb. Which is why Canada got rid of their pennies altogether.
A question less often asked concerns the export of ancient coins from Europe. Some countries, notably Greece and Italy, have strict laws about exporting antiquities out of the country, and that includes coins.
Which is why I never get ancient coins from those countries.
In fact, if you ever visit Greece or Italy and someone on the street tries selling you an ancient coin don’t buy it. They’re fake. The laws concerning fraud are far more lenient than those concerning genuine antiquities.
Rome was big. I mean really, really big, and it left its coins everywhere. Places like Britain and Croatia don’t have the same laws about artifacts, and those are the types of locations I use to source my coins.