Use and sales taxes are on new items.
The federal courts have ruled that states cannot require out of state entities (if they have no physical presence in the state) to collect sales tax for them.
They did not say that states cannot collect sales tax on out of state purchases from state residents, just that they cannot require out of state merchants to collect the tax.
Most states do not demand the tax be paid if you already paid a sales tax out of state (though some have higher tax rates and want the differenace).
To avoid everyone in a state trying to skirt the sales tax rules, use taxes have been added to the mix.
If you purchase something out of state (the state cannot require the merchant to collect for them) the state wants a 'use tax' that SURPRISE is often the same rate as the sales tax you would have paid buying from an in state merchant.
The taxes are legal, if annoying.
If you already paid a sales tax on the item their is usually a way to not pay twice.