RCW 82.08.052 Remote sellerNexus.
(1) For purposes of this chapter, a remote seller is presumed to have a substantial nexus with this state and is obligated to collect retail sales tax if the remote seller enters into an agreement with a resident of this state under which the resident, for a commission or other consideration, directly or indirectly refers potential customers, whether by a link on an internet web site or otherwise, to the remote seller, if the cumulative gross receipts from sales by the remote seller to customers in this state who are referred to the remote seller by all residents with this type of an agreement with the remote seller exceed ten thousand dollars during the preceding calendar year. This presumption may be rebutted by proof that the resident with whom the remote seller has an agreement did not engage in any solicitation in this state on behalf of the remote seller that would satisfy the nexus requirement of the United States Constitution during the calendar year in question. Proof may be shown by (a) establishing, in a manner acceptable to the department, that (i) each in-state person with whom the remote seller has an agreement is prohibited from engaging in any solicitation activities in this state that refer potential customers to the remote seller, and (ii) such in-state person or persons have complied with that prohibition; or (b) any other means as may be approved by the department.
(2) "Remote seller" means a seller that makes retail sales in this state through one or more agreements described in subsection (1) of this section, and the seller's other physical presence in this state, if any, is not sufficient to establish a retail sales or use tax collection obligation under the commerce clause of the United States Constitution.
(3) Nothing in this section may be construed to affect in any way RCW 82.04.424, 82.08.050(11), or 82.12.040(5).
(4) This section is subject to RCW 82.32.762.
[2015 3rd sp.s. c 5 § 202.]
NOTES:
Effective dates2015 3rd sp.s. c 5: "(1) Except as provided otherwise in this section, this act is necessary for the immediate preservation of the public peace, health, or safety, or support of the state government and its existing public institutions, and takes effect August 1, 2015.
(2) Part II of this act is necessary for the immediate preservation of the public peace, health, or safety, or support of the state government and its existing public institutions, and takes effect September 1, 2015." [2015 3rd sp.s. c 5 § 501.]
FindingIntent2015 3rd sp.s. c 5: "(1) The commerce clause of the United States Constitution as currently interpreted by the United States supreme court prohibits states from imposing sales or use tax collection obligations on out-of-state businesses unless the business has a substantial nexus with the taxing state.
(2) The legislature recognizes that under the United States supreme court's decision in Quill Corp. v. North Dakota, 504 U.S. 298 (1992), a substantial nexus for sales and use tax collection purposes requires that the taxpayer have a physical presence in the taxing state.
(3) The legislature further recognizes that the requisite physical presence can be established directly through a taxpayer's own activities in the taxing state, or indirectly, through independent contractors, agents, or other representatives who act on behalf of the taxpayer in the taxing state. 
(4) However, the legislature finds that because the United States supreme court has not clearly defined the circumstances under which a physical presence is sufficient to establish a substantial nexus for tax purposes, frequent conflicts have arisen throughout the country among state taxing authorities, taxpayers, tax practitioners, and courts.
(5) Therefore, the legislature intends to provide more clarity for out-of-state sellers that compensate Washington residents for referring customers to the out-of-state seller by providing clear statutory guidelines for determining when these out-of-state sellers are required to collect Washington's retail sales tax." [2015 3rd sp.s. c 5 § 201.]