Out-of-state retailers that are not "engaged in business" in North Carolina are not required to collect North Carolina's tax. However, some out-of-state retailers voluntarily collect North Carolina tax as a convenience to their customers. Out-of-state retailers include mail-order companies, television shopping networks, firms selling over the internet, and other retailers.
Essentially if you bought something online and paid sales tax for it, then you don't have to pay use tax. But if you bought something online and paid no sales tax, then you used it while in North Carolina, you need to pay use tax on the item at the sales tax rate.