Any public service company engaging in the sale of merchandise or appliances or equipment shall keep separate accounts, as prescribed by the commission, of its capital employed in such business and of its revenues therefrom and operating expenses thereof. The capital employed in such business shall not constitute a part of the fair value of said company's property for rate making purposes, nor shall the revenues from or operating expenses of such business constitute a part of the operating revenues and expenses of said company as a public service company. For purposes of this section, the providing of competitive telephone service, as defined in RCW
82.04.065, shall not constitute the sale of merchandise, appliances, or equipment, unless the commission determines that it would be in the public interest to hold otherwise.