The only requirement to advertise on the AKC marketplace is that the person not have their AKC privileges revoked or suspended.
The National Shiba Inu Club of America
http://www.shibas.org/index.html has a list of members who are breeders, and he's not on it. That's not necessarily a "bad" thing, as there are lots of good breeders who aren't on their breed club's list either, but I'd for sure want to know what he's doing with his dogs besides just breeding them. Is he competing in conformation and/or performance sports? Does he do the recommended health testing? Does he socialize the puppies to a wide variety of people and experiences? Is it in his contract that he will take the dog back at any time, for any reason? Actually, from looking at his AKC ad, it looks like the only thing he does with his dogs is breed them... not a breeder I'd be interested in...
Check out the breeders on the club website. If they don't have any litters upcoming, there is a good chance they will know of a responsible breeder who does. You will more than likely have to get on a wait list, since good breeders typically don't breed all that often (a red flag is someone with pups almost always available) and usually have a list of people wanting a pup when they do.