I think at the end of the day, when a kitten that normally is priced at $10-15,000 is priced at $900, it's just not likely to be legit. Even an F2 at $800 is not even close to reasonable. If a breeder was tempted to have a "sale" (and really they are not shoes or clothes, good breeders do not have regular anniversary sales) they might take $500-1000 off. That isn't meant as a criticism for Therrm but as a general comment for others to read when they attempt to research this scammer's name.
No reputable breeder uses Walmart money transfer any more than Western Union.
It does seem that these people were pretty good. The address does Google to an actual house (which differs from some other scammers). In terms of what you simply could ask to be more sure. When at the point that you've agreed to send money, you could simply ask for updated pics of the kittens. It's easy enough for these scammers to steal some pictures but I wonder if they remember where they copied them from to be able to source updated pics of the same kitten? Did you ask for pics of both parents? Pedigree information? I know it is for a pet, but I think that kind of question might be more likely to stump a scammer, or lead them to make a mistake. I'm a believer in asking lots of questions, if a breeder won't take the time to answer them...