RE: Not paying sales tax in Washington State
I just did a quick lookup with the State Website and found this:
*Sales to Nonresidents:*
Sales in Washington of products to be used in other states, Canadian
provinces, or US Territories that impose a sales tax of less than 3 percent
may be exempt.

I know that a few years ago when I was in retail sales the only States I
remember that could provide ID and not pay Washington Sales tax were Oregon
and Alaska.  Based on the above there may be others.  Also, if we were
shipping out of state then no sales tax was collected.  Also exempt were
sales of items that would be used at sea in international maritime trade.