There is no way the individual can prove that he did not purchase the firearms without an intention to profit.
As per the ATF:
"Determining whether you are “engaged in the business” of dealing in
firearms requires looking at the specific facts and circumstances of
your activities."
"As a general rule, you will need a license if you repetitively buy and
sell firearms with the principal motive of making a profit. In contrast,
if you only make occasional sales of firearms from your personal
collection, you do not need to be licensed."
"Courts have identified several factors relevant to determining on
which side of that line your activities may fall, including: whether
you represent yourself as a dealer in firearms; whether you are
repetitively buying and selling firearms; the circumstances under
which you are selling firearms; and whether you are looking to make
a profit. Note that while quantity and frequency of sales are relevant
indicators, courts have upheld convictions for dealing without a
license when as few as two firearms were sold, or when only one or
two transactions took place, when other factors were also present."
https://www.atf.gov/file/100871/download