If the seller has nexus in your state (meaning they have a physical presence or enough sales activity to trigger tax obligations), the seller is legally required to collect your state’s sales tax at checkout.
If the seller does not have nexus in your state, they may or may not charge you sales tax. Some sellers will collect sales tax for out of state sales. KEH used to not collect tax when I bought from them (they are in GA and I'm in TX. But now they do. Do they have a location or nexus here? No.)
In the case of the seller not collecting sales tax, the buyer is responsible for paying what’s called "use tax" to the home state, usually at the same rate as sales tax. Use tax is a “backup tax” designed to level the playing field so that buying from an out-of-state seller doesn’t mean you escape tax entirely.
This also applies to international purchases...use tax goes to the state.
Do individuals actually pay this use tax to their state? Almost certainly not, but that's the way things are structured.
This is of course separate from duties.