I've seen the local anti-gun media report a couple of times on a very popular gun shop run out of someone's home for the past several decades. They moan and complain that children live in the neighborhood, but that there's nothing they can do about it because his business is well behaved and legal. Several times someannoying group has tried to shut it down for ideological reasons, but thankfully they haven't succeeded.
This tells me that running your FFL firearms dealer business out of your home is legal in WA. Since I don't remember the county or neighborhood, I can't drill down more than that, but you have hope at least.