Originally Posted by railfancwb
Sales tax has not generally been collected on Internet sales because there are thousands (maybe tens of thousands) of different rates and degrees of partial exemption in the country.
If an online retailer maintains a physical presence (for example a warehouse) in a state that charges a sales tax on most purchases, then that online retailer must charge sales tax on any items that are sold to customers within the home state. However, if the online retailer is selling an item to a customer outside of its home state, then it is not required to collect sales tax from that customer.
In addition many states (mine, for example) require you to pay state sales tax on all out of state purchases including internet purchases via reporting on your state income tax return. If your state doesn't require this or has no state income tax or state sales tax then you don't need to pay tax.