I would abide by the law as much as humanly possible here. You know your WA state law, so follow that to the letter and you shouldn't have any problems. I have also been very curious about purchasing antique firearms through the mail as I am a Chicago resident. While I won't deviate more than this sentence, let's just say I don't even think I would take the chance given our "wonderful" mayor and I'm not even sure who the new CPD Chief is...
Buying anything online should scream caveat emptor to you. Ultimately, it will be YOUR responsibility to get as many of the facts as possible and to "pull the trigger", so to speak.
"Shut up, crime!"