visitor.us helps international visitors to the US register vehicles sales tax-free in Montana, in most cases.
In a couple of states, auto dealers are required by law to charge sales tax on vehicles that they sell.
California is one of those states. Dealers there are required to charge California sales tax, unless the vehicle is delivered out of state. You can read more about California's vehicle sales tax rules here.
Washington State is another one of these states. Dealers there are required to obtain proof of a buyer's residency in another state in order to exempt the vehicle from Washington State sales tax. You can read more about Washington State's vehicle sales tax rules here.
However, neither California nor Washington State (nor any other US state) requires private sellers (non-dealers) to charge sales tax on vehicles. So if you're buying a vehicle from an individual, you don't have to worry about any US state's sales tax rules.