Hardy,
I'm not trying to put any "bad" ideas into your head about selling black powder outside of your place of business.
First look at this picture of Jack stating on the internet that he's selling powder to reenactors on the sight outside of his premises.
Bringing Goex powder to reenactors.
On location sales help cut shipping costs for reenactors.
We would love to meet you!
Come see us when you are at your next reenactment.
http://jackspowderkeg.com/
Now what I think is routinely done is that just like there is an exemption for FFL's to sell guns at gun shows as an extension of their business premises, there is not any enforcement against licensed black powder dealers who sell at events as long as they are licensed.
Now why no one is admitting it, I'm not sure. But the ATF is not arresting any FFL dealers en mass a gunshows for selling guns off their premises under the authority of their FFL. That's because there is an exemption so that they can do it.
And if there's not an exemption for licensed black powder dealers like Jack's, then it's not enforced. Just like FFL's that attend out of state gunshows, and transfer non-curio & relic guns to in state FFL licensees for retail transfer without returning to their premises first to ship the gun back to complete the sale.
Legally they are only supposed to be taking orders for non-curio & relic guns, but it's simply not commonly enforced unless someone complains, the ATF has a vendetta against the person, or the law is otherwise broken by selling a gun that is illegal in the other state.
So try to understand, I'm not advocating something known to be illegal. If the vehicle carrying the black powder is considered an extension of the business, then the black powder magazine can be carried around to sell black powder.
Talk to other black powder dealers and ask them how they do it.
Jack's wouldn't risk his business promoting illegal activity on the public internet.
Once the ATF looks the other way for every other dealer, then they can't selectively prosecute someone arbitrarily.
The ATF has already admitted in writing that several ordinary people can buy a large order of powder, split it up and transfer it as long as it's done without profit. That's not explicitly allowed under the ATF rules either, but they don't want to prosecute the otherwise law abiding citizen over harsh rules, so they officially look the other way.
We're the people & the NRA, freedom rules and "Live Free or Die!"