The one way they could catch him is if he was buying on a 01/07 FFL and then the guns being "transferred" to his personal collection. That would be him buying tax free on the FFL and being "sold" (Transferred) on a 4473 to himself for his collection. That is 100% legal UNLESS he did not "Sell" them to himself in which he would need to pay sales tax as a retail sale.
WHAT ABOUT NOT PAYING TAXES ON GUNS BROUGHT IN FROM A DIFFWRANT COUNTRY???? LIKE CANADIA???? ALSO IF IS SELLING TO SOME ONE HE WOULD HAVE TO COLLECT TAXES CORRECT??????? WOULD HE NOT ALSO HAVE TO PAY INCOME TAZES ON PROFITS MADE FROM GUN SALES OR ANY SALE FOR THAT MATTER????????????