So for the out-of-staters, they have to audit the records of thousands of businesses hoping to find a few unreported purchases from Kali citizens? Doesn't sound very practical.
First of all, the other states will be glad to join in the search as they need the money also. Kentucky has a 6% sales tax and a 6% use tax.
Second, when most of the states join in the procedure, the investigation is not that hard.
Start off with the big businesses, the big sellers on ebay, etc. You will get a good piece of money there. Then have ebay requires that 6% (or some other percentage) on eac sale be paid directly to Ebay for transmission to the states. (That would not be hard for Ebay to do, especially how they are starting to require that all payments be made thru their Paypal system.)
Today this is made easier by the various electronic systems. Just plug in 'Clifornoa;, 'kentucky, etc and either print the list or copy it to a dvd.
Not a perfect system, not everybody will be caught, but more money than the cost of collection will be obtained.
So far, the states have not been able to cooperate. Also the fact that in some states, sales tax varies by locality has presented a problem. All of which can be corrected.