It's probably not an issue as whether or not to advertise the laws concerning muzzle loaders.
Folks who are interested in them, and will buy them, no doubt already know that.
And folks who don't know the lack of laws about them, more than likely aren't particularly interested in them anyway.
Merchandise them so that when folks walk into the store, they see the history and adventure associated with these historical firearms.
The best looking guns impressively displayed on the walls, maybe.
Lots of large posters and prints of scenes of them in use.
Scenes of the great western migration, the mountain men, buffalo hunters, Indian wars, the Civil War, the post war cowboys, outlaws and lawmen, for examples.
Include scenes of modern muzzle loading hunting and competitions from reenactor and Friendship events.
Sell the adventure first, then the gun.