It's been many years since I purchased a Canadian boat up in BC and sailed it to a port of entry where I paid US import duty, and then sailed it home to Oregon and created a title.
I do recall that sailors up there pay a BC sales tax, and it used to be around 8%, IIRC.
Chances are that your seller is trying to cheat the BC govt out of their taxes and could be even be trying stay one step ahead of having it chained up for non payment.
As to the legal "ramifications" you really need to speak to your WA registration folks directly. You can get helpful/interesting personal stories from strangers on the 'net, but nothing that will ever help you in court....
If you really like the boat and the price, one possible outcome might be to pay and take delivery in Washington waters. Then just change the title and regis. (if Washington now titles boats.)
I would wonder if that might still leave you with a tax liability if you ever come under RMCP jurisdiction, though.
While this is not quite the same as being offered 'stolen property' there is still liability attached, IMHO. Something like a tax lien
is never going away.
Note also, that it's hard to know how much trust to grant the seller, knowing that he is already a potential criminal in Canada.
, on the whole, and at the end you will not get Rebecca De Mornay to sail away with you...