Here in Colorado, we have a use tax. It is levied on the use of the personal property purchased. If you don't pay sales tax, on most personal property or the use thereof, the use tax is substituted for the sales tax that would have been paid to your home jurisdiction (city, county) in a taxed retail transaction. The state also collects some use taxes eg vehicles, rentals and such.
Alternatively, the sales tax is considered levied on the transaction. Therefore, its conceivable that the state might come after the purchaser, as well as the vendor.
Use tax on most items is supposed to be collected from the purchaser by the purchaser fessing up that they bought goods without paying sales tax. Easiest way for the consumer to stay out of trouble is just send some money to each sales taxing entity each year.
Of course, the easiest enforcement is to go after the vendor for the sales tax instead of waiting for the consumer to report their own sales or use tax liability. If all the Amazon customers were paying their Colorado sales or use tax, Amazon wouldn't have "left" Colorado.
