when a FFL in CA collects CA sales tax for a gun that has been

[Squid]

purchased by you with funds sent to the out of area seller and shipped to their address, how do they know what you paid for it?

Is that info required to accompany the gun being shipped to FFL in CA?

[DVSmith]

My FFL asks for a copy of the receipt and he keeps that with the paperwork so when he has a BOE audit he has backup proof of value. If you can't (or won't) provide a copy that he considers credible he will refuse the transfer and ship the gun back to the seller. He makes this clear up-front so there are no surprises.

[mike_the_wino]

Invoice ships with item, and those normally have the sale price on them.

If you are buying from a private party I have no idea. Never done one of those.