They'll require the business to collect it on their behalf, like states do with sales taxes now for businesses with a presence within its borders. How you justify going across the line to a business that has no physical presence in your state is beyond me. It's called interstate commerce for a reason, and Congress was specifically given control of this for a reason.
Just wait until California sales tax auditors demand to see the books of a Texas based on-line ammo or firearms retailer.