If you buy from an out of state company, you are responsible for paying sales tax (see your CA income tax form). The Internet Tax Freedom Act talks about taxes specifically to Internet sales, not taxes which apply to all purchases.
In the case of firearms, the CA business is required to collect sales tax if it comes from a business. CA BOE Sales Tax Firearms Information 495.0843, 495.0848 &295.1675.600
I hate collecting sales tax, but I have no choice if I want to follow the law. This is why I mention that if it comes from a private party and can get a letter, then I don't need to collect the sales tax.
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