I guess there are two ways of approaching the discussion.
1) you could be just looking for a pet. In this case there's really no reason to avoid mixxes. Perhaps if you want to discourage it from happening you can vote with your purchase. They would likely be healthy (although in this case it is two species mixing, by the claim in the ad, which means hybrids, which means sterile and usually short-lived, but box turtle species seem to be odd in not obeying our human-made scientific rules for species, so who knows). One issue might be that these two species occupy very different climates and landscapes in nature. What would this hybrid offspring be best suited to? A florida climate with lots of rainy forest? A drier midwest climate with winters and some plains areas?
2) you could have ideas of wanting to breed the eventual adults. In this case you should probably avoid hybrids or mixxes. If you represent them honestly for sale (any offspring of them later) you cannot name a species, only a mix and lengthy parental lineage. I know my own eastern/three-toed accidents were not very popular. I keep them and enjoy them, but they will not be bred and will live alone once they reach sexual maturity (I hope they are all the same sex, that would make it easier). People seem to (in the vast majority) be most interested in a particular species, or at least to be able to know the specific species they buy.
I'm no purist, but I now keep my boxie species separated (not only for that reason, but also so I can make sure to control the parentage of each clutch, since my PrettyBoy's babies are very popular and I'd like to have more to give out).
I would ask you, as a boxie lover and person who might eventually buy boxies again some day, PLEASE represent your mixxes fairly as well as any offspring if you take this route. I would not like to end up accidentally getting an animal that is not what I expected one day, it would be especially impactful in my constructive breeding plans.