OK, ATF site was little help. But did some Google-foo

and I believe these are the two relevant sections:
18 U.S.C. § 922 (a)(1)(A) wrote:
(a) It shall be unlawful -
(1) for any person -
(A) except a licensed importer, licensed manufacturer, or
licensed dealer, to engage in the business of importing,
manufacturing, or dealing in firearms, or in the course of such
business to ship, transport, or receive any firearm in
interstate or foreign commerce; or
http://codes.lp.findlaw.com/uscode/18/I/44/922" onclick="window.open(this.href);return false;
18 U.S.C. 921(a)(21)(c) wrote:
(a) As used in this chapter -
(21) The term "engaged in the business" means -
(C) as applied to a dealer in firearms, as defined in section
921(a)(11)(A), 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, but such term shall
not include a person who makes 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;
http://codes.lp.findlaw.com/uscode/18/I/44/921" onclick="window.open(this.href);return false;
So I guess the key here would be the legal definition of "occasional" in 18 U.S.C. 921(a)(21)(c) above. If a person holds one or more other jobs that generate the bulk of his "livelihood" and income as demonstrated on Federal tax returns, but buys and sells 52 guns per year (one per week) for a profit of less than $100 each, is that person an unlicensed (thus "illegal") firearms dealer? What about 1 gun per month for profit? What about 1 gun per week, but all at a loss? what about 1 gun per year, but at a $50,000 profit?
We can obviously get carried away with thousands of hypothetical situations. I just propose that it's not quite as clear cut as Keith B's definition above, though agree that if you admitted to the intent to "flip" a firearm, as it were, you're probably going to be prosecuted:
Keith B wrote:Yes, if you are buying it because it is a bargain with the intent of reselling only to make a profit, then you are in violation of the law without an FFL. Buying it for yourself, but then deciding it is not what you wanted, and then selling it is not illegal. Can they tell the difference? Probably not, but by the law, that is the way it works.