Generally restaurants and the like predominately sell prepared food (see discussion under Tax Information for Retailers of Prepared Food); therefore they must collect retail sales tax on the sales of all food and food ingredients sold by them. Additionally, they must pay Business and Occupation (B&O) tax under the retailing classification on all food and food ingredients sold by them.
For more information please also refer to WAC 458-20-244 & WAC 458-20-124.