I used to work for a state tax agency. I worked the fuels tax area, very black and white with rules.
The sales/use tax area would generally only attempt use tax audits on companies that were likely to buy via out-of-state supplier, and have that hard physical asset sitting in the business. It's pretty easy to track dentist chairs, medical equipment, things like that.
They never went after Joe Consumer.
What's funny is, the state had an automated system that allowed a taxpayer to voluntarily remit use taxes when they filed their taxes on-line. No one ever used it.
A tax agency employee actually found out the line had a coding glitch in it when he attempted to voluntarily pay use tax on items he'd ordered from out of state. The line basically blew up the tax return and would not allow further processing. It was an embarrassment.
You are correct. Enforcement would be difficult, at best.
The state would basically have to operate under the assumption that, unless the consumer was in business, that most items ordered from out of state would be taxable to the consumer.
It would be on the taxpayer to have records to prove he or she was not the final consumer. That gets into a real legal mess. Unless the state can force retailers to provide customer lists and purchases for taxpayers in that state, the state simply has no standing to go on a fishing expedition.
Maybe not in your neck of the woods, but they sure did in my state about 25 years ago.
Sales tax receipts in New York were always a big issue for a number of reasons. A big one was that counties and cities have the authority to impose their own sales taxes on top of the state sales tax. The state sales tax rate for New York is only 4%, but when you add city and county rates on top of it New York City has one of the highest sales taxes in the country at 8.875%. New York City is within an hour's drive of three other states (New Jersey, Connecticut and Pennsylvania) with lower sales taxes, so they were always losing sales tax revenue when NYC residents would drive to other places and go shopping there.
In the early 1990s, it got so bad that the NYC government had a brilliant idea to send tax agents to drive around the parking lots of shopping malls in neighboring states. They'd record the New York license plates of cars parked there, then use the information to do audits and attempt to collect the "use tax" from these people.
Someone in the New Jersey governor's office got wind of this, and the state attorney general issued a public statement informing the NYC mayor's office that these out-of-state tax agents had no jurisdiction to operate in New Jersey. He also threatened to have the tax agents arrested for trespassing and charged as if they were sexual predators stalking potential victims. That put an end to the whole practice very quickly.