Wash. Rev. Code § 51.16.200
Whenever any employer quits business, or sells out, exchanges, or otherwise disposes of the employer's business or stock of goods, any tax payable hereunder shall become immediately due and payable, and the employer shall, within ten days thereafter, make a return and pay the tax due; and any person who becomes a successor to such business shall become liable for the full amount of the tax and withhold from the purchase price a sum sufficient to pay any tax due from the employer until such time as the employer shall produce a receipt from the department showing payment in full of any tax due or a certificate that no tax is due and, if such tax is not paid by the employer within ten days from the date of such sale, exchange, or disposal, the successor shall become liable for the payment of the full amount of tax, and the payment thereof by such successor shall, to the extent thereof, be deemed a payment upon the purchase price, and if such payment is greater in amount than the purchase price the amount of the difference shall become a debt due such successor from the employer.
No successor may be liable for any tax due from the person from whom the successor has acquired a business or stock of goods if the successor gives written notice to the department of such acquisition and no assessment is issued by the department within one hundred eighty days of receipt of such notice against the former operator of the business and a copy thereof mailed to such successor.
RCW 51.16.200