the problem is, if your someone who buys 3 guns a month, then waits till the next month to sell htem off, thats a business.
While there are transactions that are obviously "private", and other transactions that are obviously "business", there's no "bright line" separating the two - you have to look at the totality of the circumstances.
I can think of reasonable examples where someone might buy three guns one month and then sell them the next month without them being "business" transactions. Maybe the credit card bill arrives and the buyer realizes he's outspent his budget. Maybe he needs to raise some quick cash to cover unexpected medical expenses. Perhaps after buying the guns, he comes across a once-in-a-lifetime opportunity to buy one of his "bucket list" guns, and sells his recent purchases to fund his dream gun.