Here in Washington State you are pretty much stuck with paying the use tax,
which the state requires the buyer to pay.

There are some loopholes, but they may not work for your RV-6. An airplane
purchased for resale buy a licensed aircraft dealer is not subject to use
tax. An airplane purchased for use as a leased aircraft or rental is also
not subject to use tax, but it must collect sales taxes on all the lease and
rental income unless that rental income is in conjunction with a service,
such as flight instruction. Hence, student pilots renting a plane for dual
instruction do not pay sales tax, but student pilots flying solo have to
pay.

Airplanes purchased for the purpose of interstate commercial transportation
are also exempt (the "Boeing" clause), as are airplanes purchased for use
outside of the state.

The use tax is supposed to be on the "fair market value" of your aircraft,
not necessarily what you paid. Any trade that you made for the aircraft is a
deduction against fair market value -- you pay use tax only on the
difference between the trade and the new airplane. The listed purchase price
on the FAA sale agreement is supposed to be evidence of the fair market
value, however. Obviously, if you buy the aircraft for "$1 and other
consideration," then the state is probably going to claim that your fair
market value is higher. But if you dishonestly put down some amount at the
low end of the value of your airplane, you will probably get away with it --
at least in this life (do tax cheats go to Hell?).