As others have said, it varies a lot by state and locality.
In California, the state levies a sales tax, and so can counties and municipalities. The State Franchise Tax Board, who issues licenses to collect tax to businesses of all sizes, also collects the money taken in via sales of all kinds.
The actual percentage thus varies quite a bit; in Santa Clara County, where I live, it's 8.5 percent and likely to increase within the next year.
The state keeps everyone honest this way: depending on your volume of tax collected, you file a tax return and send along the collected monies quarterly, monthly or weekly.
And yes, it applies to used and consignment firearms, just as it applies to any other used or consignment item for sale, including auto auctions and the like.
In dealer to dealer transactions, the buying dealer buys wholesale, but the selling dealer (of any good or commodity) must provide the buyer his resale number in order for the buyer to avoid paying sales tax. Note that the buyer in this instance is the entity that will sell the item retail.
I had such a resale license at one point with a business I had some years ago, and will never do so again. It is a royal pain and complete time-inhalation exercise to fill out the tax forms and rebate the money to the state.
To top it off, I was audited by the IRS one year and actually had a supervising IRS auditor ask me what the payments were that I was making to the state! (This even though the state resale number was included in documentation on the IRS return. Just another reason I so dearly love mindless, incompetent bureaucrats...
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Bill
(Rant over.)