This information right from the ATF website
(21) The term "engaged in the business" means—
(A) as applied to a manufacturer of
firearms, a person who devotes time,
attention, and labor to manufacturing
firearms as a regular course of trade
or business with the principal objective of livelihood and profit through
the sale or distribution of the firearms
manufactured;
(B) as applied to a manufacturer of
ammunition, a person who devotes
time, attention, and labor to manufacturing ammunition as a regular
course of trade or business with the
principal objective of livelihood and
profit through the sale or distribution
of the ammunition manufactured;
(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
As I have always understood it, you can buy and sell as many firearms as you want so long as it is not your primary source of income I currently don't have an FFL but I have had one many times over the last 30 years and have spoken to ATF agents regarding this issue and that is what I have been told . As a rule I wont sell any gun I buy at a gunshow at the same gunshow and If you only sell guns to advance a collection or in a hardship having to pay for an unexpected bill makes no difference if you are selling it for more than you paid for it