Here’s what ATF says:
May I sell black powder without a license?
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No. Anyone who engages in the business of selling black powder, regardless of quantity, must be licensed as an explosives dealer. [27 CFR 555.41(b)]
Note that it specifically says engages in the business. If you advertise, repeatedly sell, especially at a profit, then buy more to sell, or have a reputation of being the go-to for BP it might be difficult to claim you’re not engaged in the business, but I wouldn’t worry about letting a spare go to someone you know at the price you paid.
Of course this is just my opinion but if you search the internet you will find that “engaged in the business” has been core to some firearm cases.
Here’s what ATF says about being engaged in he business:
“The term “engaged in the business” as applied to a dealer in firearms means a
person who devotes time, attention, and labor to dealing in firearms as a regular course of trade or business with the principal objective of livelihood and profit through the repetitive purchase and resale of firearms.”