OP, this IS a puppy farmer, or at best, a retail outlet for one. What happens is that the volume puppy farmers (typically with between 30 and 50 breeding bitches) churn out pups. These are then sold as wholesale items to retailers, either the pet trade, or to people who act as a "front" by having the pups in their home. A number of MNetters have been caught this way.
The fact that he's previously sold Yorkies (yet another PF regular breed) simply confirms this, and I've just had a quick Google and if he's the chap currently advertising on Preloved, then the fact he is offering to deliver pups is also a HUGE red flag.
Puppy farming causes untold misery, both to the poor bitches involved and to the pups, who are often unhealthy, poorly socialised and have not had the proper health screening that other posters have described.
The other particularly nasty thing about these cross breeds, is that the KC have a limit on the number of litters that can be registered for a bitch, as a measure to protect their welfare. So a pedigree CKC bitch can only register a limited number of litters, and there is an upper age limit. What unscrupulous breeders do to get round this, is use a KC bitch for one of these "designer mongrels", mate her with the poo/chon of choice, and hey presto, a valuable litter. Because she's produced a cross litter, the pups can't be registered with the KC, but the breeder can proudly say, look, Mum's a registered KC bitch. This way they can churn out extra litters and get round the limited welfare protection for the bitch. Most puppy farmed bitches end their lives on the farm when they've been finally knackered, with only a tiny handful making it out to rescue places.
It also strikes me as a particularly horrible irony that much of the puppy farming trade is driven by mothers wanting pups for their children, yet to do so, they make mothers of other species live a life of misery.