Straight from the horse's mouth, Page 6:
What does it mean to be "engaged in the business of dealing in
firearms"?
Under federal law, a person engaged in the business of dealing in firearms is 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." Under federal law, conducting business "with the principal objective of livelihood
and profit" means that "the intent underlying the sale or disposition of firearms is predominantly one of obtaining livelihood and pecuniary gain, as opposed to other intents, such as improving or liquidating a personal firearms collection."
Consistent with this approach, federal law explicitly exempts persons "who make occasional sales, exchanges, or purchases of firearms for the enhancement of a personal collection or for a hobby, or who sells all or part of his personal collection of firearms."
Do I Need a License to Buy and Sell Firearms