It's not illegal to sell the old documents. Sellers of that sort of thing usually specify that they're for collection/nostalgia purposes only. And that's not illegal.
It is of course illegal to use them to register a car, but it definitely happens. Especially on the old cars. Cars from the 30's and 40's especially only have 2 or 3 identifying numbers, and some of the earlier ones were even stamped by hand. They weren't VIN numbers either, most were just serial #'s. A lot of the serial # tags were attached with screws too, so those can be lost (or switched!) with ease. It doesn't take much to replicate either of those.
But when the smog laws dictate that you register your 1930's hot rod as a "new construction" because of too many new parts, I can see the frustration. In California, only so many "special construction" cars can be smog exempt each year (500 I think), and builders usually camp out at the DMV the night before the numbers reset. Special construction rules vary greatly from state to state, and some of them are pretty ridiculous. California, believe it or not, isn't the worst of them. You can usually get a title with an inspection here, even if not all your numbers match, as long as you can demonstrate that everything was actually from the time period- ie, different body/frame #'s on your model A, but all original steel. I have a '56 Austin Healey with a state assigned VIN because the "VIN" tags (which are serial # tags on that car) were removed for painting. No rivets mind you, they're attached by regular old sheet metal screws. But of course those inspections depend a lot on the inspector, and not many folks want to roll the dice with a vehicle they sank tens of thousands into.
Later cars, like ours, have VIN's with multiple stampings, "hidden" #'s, etc, and are harder to re-body. To me, its also a different level of deception to do it. Early cars were never assigned VIN's like that, and registration was a whole different ball game. Those numbers were not intended to be used like they are today, and counterfeiting wasn't a problem (or no one cared) back then. I mean, not all PEOPLE had birth certificates in that era, cars were a minor detail.
But the later cars had an established system to track them (VIN's), and were stamped specifically so that they were hard to copy/duplicate for purposes of identification. But you still better believe that a bunch of those big dollar hemi cars etc are re-bodies. They HAVE to be, given what some of them looked like before restoration. But even that's semantics. If you replace the entire car around the VIN's with reproduction metal, is it a re-body? In many cases, its not (or not illegal at any rate). Now that you can buy pretty much the entire car, that's not as hard to do anymore. But it is a re-body if you cut the VIN's out and put them on another car. Either way, the car that left the factory is pretty much gone, the difference is only whether another factory car died to make it happen.
