There are no laws proscribing the owning or buying of any coins in the United States.
There may be laws, especially local laws and ordinances, prohibiting the open display of Nazi memorabilia, but simply owning them isn't illegal. Enforcing an outright ban on sale or ownership of Nazi coins in the US would certainly be problematic, not least because of the large number of WWII veterans who souvenired a Nazi coin as a war trophy during their time in Germany. And there may be restrictions on import of certain coins (particularly ancient coins; I don't think there's currently any restriction on importing Nazi coins) into the United States. But if a coin is already in the United States, it can be offered for sale there without restriction.
The law in Germany prohibits the "open display" of the swastika. For coin dealers, so long as the Nazi coins are placed swastika-side-down in the display case, it's legal.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis