Royal Tartan - only made with permission from Her Majesty The Queen
Kilt: Cannot be made without permission from Her Majesty The Queen.
Kilt: Cannot be worn without permission from Her Majesty The Queen
It doesn't say you can't sell an existing kilt in this tartan, though.

So sell it to an American. Heck, an American might pay more for it if it's "restricted". Include the letter from Buckingham palace, as proof. Americans love that kind of stuff. Double your money!