Buyer X is still responsible for "USE Tax" in the state of Washington for all purchases, regardless of where the items were purchased. Now, does that mean people actually PAY the Use tax, which is really based on the honor system? HELL NO THEY DON'T!
For example, you live in Washington and you buy a TV in Oregon (no sales tax) for $1,000. You pay the retailer in Oregon $1,000 and say "thank you!". Technically, you owe Use Tax of $95.00 (based on where you live, this is based on Seattle.) You are supposed to fill out the Use Tax form and send a check to the Dept of Revenue.
Like I said, do people do it? Sometimes, but mostly it is ignored.
More than you ever wanted to know about Use Tax in WA:
http://dor.wa.gov/content/FindTaxesAndRates/UseTax/
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Scott
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