You need to get a Master Business license, then a City Business license (if you live in a city), or a Residential Business license (depending on city/county), or Administrative Conditional Use Permit (depending on city/county). Then you get your FFL (Federal Firearms License) as either Dealer or Manufacturer (depending on type of business you intend to do). Finally, after getting all that you can get your State Firearms Dealers License.
You must have all these to conduct any business in firearms in Washington. All business must be conducted at the location listed on the license with the only exception being Gun Shows.
See 9.41.100 and 9.41.110 for the laws concerning operating a business in firearms.