Do you mean by not collecting tax meaning to not follow the law? If the firearm comes from a private party and is an occasional sale, then sales tax does not need to be collected. Otherwise it does.
It seems a bit strange to me that you might want to find a FFL who is breaking the law as that could cause future problem for you.
FFL Transfer/Special Order Dealer since 1993.
Net-FFL list maintainer.
Never try to teach a pig to sing. You waste your time and you annoy the pig. - Robert A. Heinlein