You can buy black powder on line in any increment from 1 to 50 lbs depending on the supplier. Here's a chart from a survey I did of internet based retail suppliers back in February. The prices are delivered at your door, so they include hazmat fees and delivery charges:
In order for you to sell it you will need an excise tax license from BATFE; I believe is $300/year. Storage requirements are locally set, usually by either the fire marshal or construction codes. In general you will need a lockable magazine capable of storing up to 50 lbs. Insurance requirements are negotiable and vary widely. I've heard many people say the hassle isn't worth it. I find the argument specious, but I can't fault it; I don't sell it either, so who am I to say someone else is being truculent. But then, I'm not trying to run a retail business, so I must be an expert....