They have enacted e-commerce laws that require the tax be paid either by the seller or the buyer. The best thing to do is check with your states sales tax office.
The sales tax (legally called the use tax) is *always* paid by the buyer. The seller only facilitates the tax collection when required.
But you are correct, some states have a reciprocal agreement. I live in northern Massachusetts which has a 5% sales tax and frequently go to New Hampshire to shop, about 20 miles from home. If I buy a big-ticket item that needs to be delivered, then the seller `knows` from the purchase that I am using the product in Massachusetts and is required (because the store also has locations in Massachusetts) to collect the sales tax from the customer.
EVERY state is a little different. Your best bet is to look on your state`s official website and look up "sales tax" or "use tax".
MannMade Digital Video