Without doing any research, Cockerel, the essence of "selling" or a "sale" does not necessarily involve money, currency, gold or anything in particular, except that it involves something of commercial value given in return for a good (services are not typically subject to sales tax, or at least, they didn't use to be, because generally, selling services at a profit generates in income tax).

So if I "sell" you an ash tray, and you give me something of commercial value as payment, then I believe the sales tax applies to that transaction, in an amount of the tax times the commercialy reasonable market value of the consideration transferred. If you've given me a ten dollar bill, it's pretty simple, if you've given me a gram of gold, it's pretty simple, if you've given me financial advice, it's not so simple, but still, theoretically, a value can be placed on it. If you've given me a bushel of corn, it seems we have a good for good barter and both should pay sales tax! What they used to call and perhaps still call, "the underground economy" along with payment in cash for services, where the person accepting the cash has no intention of claiming it as income for the services.

Just musing.