If you ask that man why he has to charge $17-18 a pound for black powder, you will learn about the expenses he incurs just to be able to store and sell it which he can only pay if he sells the powder at that price. And, he is not in the business for his health. He has general overhead and a need to feed his own family, too. He can buy the powder in volume( so can you) at about $12.00 per pound. But he has annual State and Federal licenses to pay, has to maintain logs, and a powder magazine that meets state and federal standards, that isn't free, all before he figures out if he can sell the powder quickly enough for a price that pays him for his trouble in carrying it.
If you have a local Black Powder club, it would be in both the member's interest, as well as the local gun store owner if you helped him construct the powder magazine, and your club fronted the money for the annual fees. Then, he could find a way to let you all buy powder at a much better price, than he charges other customers. Of course, this would only continue as long as he sees that the club members are also buying other products they need from him, too.
But, if a group of shooters would " adopt " a gunshop, invite the owner to use the club facilities, and attend club parties, you might be surprised at what a deal he makes for members of the club. And, if other shooters who are his customers find out they can get better prices by joining your club, everyone benefits.