That's a really good question. Your answer is that we did some quick research for you on the Form 1040 Schedule A (itemized deductions), and find that there is no specific mention, or discussion, of sales taxes paid on out-of-state purchases. There is, however, a good deal of treatment for situations where a taxpayer moves between multiple states during the year, as well as when the taxpayer moves between local sales tax districts within a single state during the year (an in-state move, essentially).
We also note that the companion income tax deduction, for state income taxes paid (which can be taken in lieu of the sales taxes paid deduction, as a taxpayer election), allows for deducting state income taxes paid to more than one state in a given tax year.
As such, and absent any rule or law to the contrary (of which we cannot find any evidence), it seems quite reasonable to conclude that a taxpayer can indeed claim the sales taxes paid on a motor vehicle purchased outside of their home state, when otherwise claiming a state sales tax deduction for purposes of itemizing.
Therefore, your answer should be yes: you can include the Washington sales taxes paid as part of your overall itemized deductions.
Thank you for asking this interesting question.
