There aren't many checks and balances that can be applied to an order of a data decal*. Just like a title service, so long as the owner of a vehicle can prove legal ownership and provide documentation and photos to show that said vehicle exists and is in that owner's possession - whether it is Swiss cheese or not - that's enough to get the decal produced and sent to the buyer's door, regardless of whether the vendor acts in good faith or not.
Of course, the buyer may go slap it on a completely different car. That's the problem, and there isn't any way to check that. Case in point: A friend of mine owns (possibly "owned") an original, never-used Chrysler broadcast data sheet (either 1968, 1969, or 1970; I do not remember which). If everyone's intentions were genuine, this paper would have little value except for being framed (after all, even a crumbly original data sheet carries more historical provenance than a mint recreation printed on original paper, even if all the data is genuine). In short, it shouldn't be an ultra-desirable piece. Well, if you only knew how many people have begged, screamed, and pushed the envelope of human tolerance for him to sell that paper at any cost, you'd take a very dim view on a number of very "respectable" Mopar folks. You know what they had planned for that build sheet.
*Far as I know, the OP does not offer reproduction VIN plates on his site, only door jamb decals. Of course, threads like these could also be seen as a back door advertisement for just that, but that's another story.