You raise an excellent point of course no conservative can ignore, however I do believe there might be problems with selling the coins piecemeal. Especially the ultra-rare one worth a million dollars.
There will always be questions as to proper chain of custody when selling coins of this value. No pawn shop or other broker is going to just buy them no questions asked. And ID will be required, etc.
In the end, some racket is going to get their cut, either the IRS and their related state goons, or Jimmy Broken-nose at your local pawn shop. There is no getting around that loss.
Maybe Jimmy Broken-nose wouldn’t take as much as the IRS however.
Pawn shops are pikers. These are going to serious numismatists, both individual and maybe some coin shops who actually know something Bout them.