The rules on those sort of things have to do with explosive devices. So you can sell all the gun powder you want, but if you drop a wick in the top of the can, you are breaking the law.
A buddy of mine almost got in trouble because of something of the sort. He reloads shotshells, and had a glass tube with powder, wad, and shot to show the setup. The police came to his house because of some dispute, and they tried to make some trouble over the tube, because it could appear to be a home made explosives device.