Re: Is this even legal?
Nothing wrong with collecting firearms then selling them for their increased value.
Now if he is buying 10 at a time, then selling them the next month...it could be questioned. But, questioned is about it. As above mentions, if it's a primary source of income...you're screwed.
I've actually done this as a hobby. It was about 15-20 years ago but I probably went through 30 guns a year for a couple years...it was a joke "what gun does Sean have today/this week". I Spoke to an ATF agent at the time and his question was..."what's your day job?"...me- "gunsmith", "OK, Does that job pay your bills?"..."yes" "You're fine"
all depends on volume vs sustained income...and the interpretation of the ATF agent questioning you.
I know laws change over the years so things may be different now. If some agent wanted to make a sink, they probably could