Well, there are two problems with a paternity/maternity test:
1- If I find out after the fact that it's not from the father they say it is, and they know that they could be "busted" by such a test, don't care, and still sell fraudulently, it is a ridiculous amount effort involved in trying to rectify things, and I would not trust that I'd ever see the money back even with all the effort having been expended. I appreciate Lucille's optimism in these matters, but you cannot (legally) physically force someone to pay even with a judgement against them.
2- In order to be able to run a paternity/maternity test, the seller already has to be a participant in the process, since they are (presumably) in possession of the parents. This is still easy enough for scammers to scam, since the people running the testing lab will never have physical access to the specimens in question. In cornsnakes there is a much simpler solution that is no more open to fraud, and doesn't cost $50 per animal... here's an example:
http://www.herpregistry.com/acr/Registry.php?idnum=81
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As far as a genotype test, that would be many many times more valuable for breeders than a paternity test could ever be for buyers. At this point it would be stupid for me to cross a quintuple het for amel, anery, motley, hypo, and lavender to, say, a triple het for diffused motley charcoal, because I would get a bunch of hatchlings that were basically "normals and motleys, plus you gotta try to figure out which, if any, of the other 6 genes they're het for before you could even plan anything, plus you'd have to keep a bunch of them for 3 years just to figure out if any of them are even worthwhile."
(Yes, these are real-life examples, and most of my snakes are het or homo for 3-5 mutants.)
I also have a snow motley that is possible het for lavender, and possible het or homo hypo. I would like to make hypo lavender motleys with her, but I don't even know if she carries the right genes. It would be worth at least $50 to know her genotype today, so I can decide whether or not to invest another 2 years into raising her up.
It would be a great advantage to be able to use hets as breeders with the knowledge of the genotypes of their offspring. You could produce morphs based on recessive traits with the same breeding strategies used for dominant genes. That's a huge deal.