As one who has not reloaded ammunition in a number of years, I’m going to guess that the only reason Amazon won’t sell you black powder (or gun powder) outright is due to shipping restrictions. Which means instead of buying the ingredients with a highly-trackable transaction from Amazon, your low-budget terrorist will have to resort to buying the powder pre-made, and anonymously in a cash transaction from the local gun dealer. Last I checked, they’ll sell you big cans of the stuff for cheap.