Well thats simply untrue, but I guess you can say anything on the internet.
I can list at least 10 states off the top of my head, that do have a sales tax on boats. Can probably find another dozen states in a few minutes. And I'm sure there are plenty more. Tax will be collected when you title it. And you don't have legal ownership of the boat until the title is in your name. You can call it a sales tax, or a titling tax. The same goes for any item with a title, car, mobile home, cargo trailer. Some states even require a title for an outboard motor. And let's not forget about the states that have a personal property tax on some boats. A title proves ownership. Registration allows the item to be used on the water or the roadway.
List them and we will research this together. Other vehicles are irrelevant to a discussion on common practice with boats. The states that bill vessels under property tax like Deleware are an exception also. USCG documented boats also typically require no tax to register at the federal level so many folks (especially large yacht owners) go this route and then register them in a state/principality without tax.
I can assure you in FL (6% sales and usage tax on hull value ONLY due when titling) and MD (5% excise tax on hull value when titling/registering) that what I described is common practice. I have even met used boat dealers that do not title and register their vessels for this very reason. To say they don't own them is laughable when they're collecting large checks for these vessels that they're moving in and out within a matter of weeks.
And you're dead wrong about the title proving ownership...all you need for that is possession of a signed and notarized title and notarized bill of sale in your name. I've bought and sold 10+ boats and have never once had an issue and no one that has ever bought a boat from me has either, including one large 38' sailboat I sold that the new owner decided to have USCG documented. Most states give you 30-90 days to register a boat anyway and there are typically no penalties and plenty of grace. The laws for this are fairly lenient as they should be.
I would never not recommend paying the tax on a vessel and not registering it. What I was saying before was for certain project boats/parts boats that won't see the water for years--if ever again. When itemized properly, vessel sales/usage/excise tax is typically not even that much and though you can purchase insurance without the boat titled in your name, if a claim were to arise it could make things complicated.
As for why the OP is finding this to be true, I would be leery and want more information from the sellers. If they have a signed over title or registration (for states that don't title) that is addressed to the seller, or left blank which is common practice, then it should be okay. But if the title/registration is not signed over, then transferring ownership would be impossible. Are these real titles or are they copies OP? The real ones look like the paper money is printed on.