In Colorado you only pay sales tax on the difference between the 2 cars.
Car is $20,000
Trade is $10,000
Taxable amt $10,000
Calculated at 8% you would save $800 so if you cannot sell your car for more than $9,200 than you might as well trade it.
I would check with state of WA to see if they do the same, IIRC most states do only a few don't.