Wash. Rev. Code § 69.38.030
It is unlawful for any person, either on the person's own behalf or while an employee of another, to sell any poison without first recording in ink in a "poison register" kept solely for this purpose the following information:
The purchaser shall present to the seller identification which contains the purchaser's photograph and signature. No sale may be made unless the seller is satisfied that the purchaser's representations are true and that the poison will be used for a lawful purpose. Both the purchaser and the seller shall sign the poison register entry.
If a delivery of a poison will be made outside the confines of the seller's premises, the seller may require the business purchasing the poison to submit a letter of authorization as a substitute for the purchaser's photograph and signature requirements. The letter of authorization shall include the unified business identifier and address of the business, a full description of how the substance will be used, and the signature of the purchaser. Either the seller or the employee of the seller delivering or transferring the poison shall affix his or her signature to the letter as a witness to the signature and identification of the purchaser. The transaction shall be recorded in the poison register as provided in this section. Letters of authorization shall be kept with the poison register and shall be subject to the inspection and preservation requirements contained in RCW 69.38.040.
RCW 69.38.030