Re: proof of paid sales tax
USCG documentation does not preclude your state of residence from requiring you to register and keep your registration up to date. Local Law enforcement can ticket you for not keeping a current sticker on your boat - but they can not require that you display your numbers on the boat (if you are USCG documented).
Of course to register you have to pay either a sales tax (legally only if new) or an excise or use tax (depends on the state). They can even hit you with a use tax if you move from one state to another - although most only do the difference between what you've already paid and what they charge percentage wise.
That they (state tax collectors) know you MUST have paid if you are registered doesn't preclude them from being bureaucratic idiots and insisting you produce a 15 year of receipt.
In the event of moving from one state to the next generally you can contact your registered in state and have them print you out a proof of payment document. You'd be silly not to take that with you. Funny, they don't do the same thing with cars - I guess enough lawsuits have taught them it's a fruitless tree.
I"m curious as to what happens when a cruiser truly detaches from land and has no fixed address. How do they maintain a state registration?
Lessons learned are opportunities earned.