KC Assured breeder here.....a reputable breeder would never ask for money up front, and the cash element implies that they don't want a paper trail (ie no tax) so it's untraceable.
Are the parents health tested? Cavaliers are known for having significant health issues and have health tests that are mandatory for KC breeding - regardless of the puppy being a mix (yuk) it should still have the health tests done. I cannot stress this enough, I bought a pug years and years ago who I thought was from health tested parents and it turned out she wasn't, and then she died at 13 months old from an inherited disease so please save yourself a load of heartache and do your homework.
The bare minimum a puppy should be sold with is vaccination, microchip and insurance - insurance is free for the breeder so there's literally no reason it shouldn't have it. The mum should be available, dad maybe not because they've used a stud.
Even if it's not KC registered, if they've got pedigree parents then you should be able to have a pedigree (all a pedigree is is a family tree with the names of all it's relatives). I sell my puppies to homes where I've had chance to get to know the new owners and their puppy packs included a weeks worth of food, bed, bowls, health record, certificates of everything (parents health testing and a copy of the KC standard so they have record that it's all been done), pedigrees, care sheet, contract, toys, etc....
Personally I wouldn't touch it with a bargepole. 5 stars means absolutely zero, and in 35 years of dog showing and breeding I've never once advertised a puppy, and especially not on Facebook. I'll also add that I only ever breed when I want a puppy for myself and I sell the rest, so it's not very often but I want them to be perfect when I do! Good breeders care immensely and aren't breeding for money.
Also, go on the KC website and go through the breeds - you'd be amazed at how many dogs are allergy friendly. Crufts is in less than a month, you can see all the breeds on the TV and get and idea of what you like the look of too.