That way no one accuse you of intending to engage in illegal trading.
You don't have to sell at cost or for a loss. There's no problem in making money on a gun sale, the problem is when you make it into a business.
Buying a gun and later selling it for a profit is fine.
Buying several guns and selling them all at a profit can get you into trouble. The more guns involved and the closer the purchases are to the sales the easier it will be to prosecute you and the more likely you are to eventually attract attention.