I’m just about to pickup a used MS in Vegas but live in California. You have to pay sales tax for any car registered in CA. Either the seller collects it or CA DMV gets you when you register the vehicle. There are several fee calculators on the CA DMV website to help you calculate the fees.
DMV will not “double tax” meaning that if you can show you paid tax on an invoice or something, they reduce the amount you owe by that amount. If you are buying from Tesla like me, they simply charge you the tax based on where the vehicle with be registered. Private party, probably wouldn’t charge tax, but then when you register with DMV they will tax you based on what you paid and the sales tax rate in your registered county/city.
The fee calculator are very accurate so just google it and you’ll be fine.