Don't think PA is a member of SSUTA - Request they provide you with a copy of their PA tax registration.
North Carolina [despite all the other ways the legislature has screwed up] has a very reasonable solution to sales tax on out-of-state purchases. Any untaxed transaction over $1000 you have to report and pay the tax of about 6%. On transactions under $1000, you have two choices: keep track of every purchase, and pay the approximate 6% tax on the total; or pay a "safe harbor" amount based on your gross income - about one-tenth of one percent of gross income [that is, they figure taxpayers spend on average about 2% of income on untaxed out-of-state purchases.] So if you spend more than 2% on untaxed out-of-state purchases, you can come out way ahead. ["Safe harbor" means that if you pay that one-tenth of one percent of gross income when you file your state income tax, they can't come after you for more use tax even if you spent 100% of your income on untaxed out-of-state purchases.]
Yes indeed. Heritage collects sales tax for California. However, California exempts coin purchases from sales tax if the amount shown on the invoice for however many coins is $1500 or more. The supposed reasoning is that up to $1500 the buyer is just playing, and at $1500, it becomes an investment. Really just penalizes less wealthy collectors but not wealthier collectors. If possible, it pays to buy $1500 worth in an auction. Worst case would be an invoice for $1499. Sales taxes are regressive, meaning that poorer folks pay a higher percentage of their income in such taxes compared to wealthier folks. The Calif law on sales tax on coins makes it even more so for collectors. Cal
Use taxes are likely to be coming to a lot more state income tax forms. I think New York was the first to start asking all filers to declare the value of goods they purchased over the internet from out of state - if you lie, then you're engaging in tax fraud.