BATFE is deliberately vague on what constitutes doing business. Is buying 5 guns and selling 4 OK? Is buying 10,000 guns and selling 9,999 OK? The difference is, after all, only a matter of numbers. And if you sell those 9,999 to people who want to avoid filling out a 4473 and going through the NICS check, are you an innocent collector "improving" your collection or an unlicensed dealer?