From what I remember:
If you buy from a private party out of state, then the tax will be charged based on purchase price.
If the firearm is a gift from someone out of state, then the tax will be based off the blue book value
If the firearm is part of a will or estate, with official documentation, then no tax will be charged
If the firearm is purchased from a retailer, then tax will be charged based on invoice
If you paid sales tax to another state when you purchased, then you will get charged for the difference based on tax rates. For example, if you paid 5% in another state, then the FFL in Seattle would charge you 4.5% tax.
Pretty much, there isn't a way to get around it other than transporting it yourself into the state. At that point, it is up to you to pay the state use tax.