The regs mention specifically that you are not to buy and sell with livelihood or income replacement as a goal. I have sold many C&Rs but then again, me selling 10% of my collection would be me selling 100 guns
In the end, they could just take your license away, but for prosecution, they obviously need to have a strong enough case to assume they win. However, since its vague, they likely would have a tough time winning a case unless you were buying 50 model X rifles and selling 48 per year, which then is pretty obvious. The people that suggest you have to sell to break even aren't being realistic. The regs do not say its illegal to sell a gun for $250 that you paid $200 for, as long as you are bettering your collection. If you had employees or a store front, but a C&R, then you would get nailed because at that point, you are attempting to draw income, and thus making a business of the C&R. Its important to understand these distinctions, and to not use ignorant blanket statements.