Most states in my experience will charge you sales tax on a vehicle if you move within the first 6 months of ownership. You can get around this if you keep the initial state's registration for 6 months, so all you have to pay is registration in the new state. That's a risk because you're typically required to update your plates long before that legally speaking, unless you're military. It gets complicated because the laws vary with each state, but most will only charge you the difference in tax. WA and CA have similar sales tax rates, WA probably a little more since they don't have income tax, especially if you're moving near Seattle/Bellevue. It's around 10.2% if I remember correctly.