From the ATF
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.
From whitehouse.gov
Repetitively offer for sale firearms that are like new in their original packaging;
This describes every handgun I have that wasn't inherited.
What is repetitively, daily? Weekly? Monthly? Yearly?
I have bought guns that I thought I wanted, taken them to the range, put 150 rds down the barrel and promptly posted them for sale here. Turns out, I really didn't like those guns.
Some guns appreciate in value, if you bought a gun, it gets some cult following and suddenly it is worth 3k more than you paid for it, were you in the business? What if it is just higher because of inflation? Does that have to be argued in court? Will every class 3 machine gun owner be in the business because his hk sear went from 2k to 45k or his mac 9/10 is now worth 12k when he bought it for 2k.
It doesn't have to be drastic to be worrisome. It just has to leave enough grey area where it was cut and dried before.