Washington state does not have a requirement to register firearms. In short, you don't register guns there. Would suggest you visit a local gun shop for information on Washington State and firearms.
No, it is not illegal to not get a pay stub in Washington state. Washington is one of the few states that does not have pay stub state requirements.
Washington State does not require registration of ordinary Title I firearms.
Potato guns leagal in Washington state
yes
Laws vary from state to state. In MOST states, there is no "registration" of firearms as such. If you live in a state that DOES require registration, then yes, that would be illegal.
All firearms are legal when they are manufactured. Illegal comes into play when you start looking at federal, state and local laws.
Yes. It is not illegal in the state of Washington. But it is in 14 other states.
yes
yes
yes
They're not.