To be as kind as possible...and I've seen it happen...It is possible the seller is just very misinformed. Some people buy birds from other people without understanding anything about breeds, and misinformation multiplies.
It is possible this was from parent stock that originally were Silkie-Cochin mixes for somebody's "sizzle" project, which is a "frizzled" Cochin (curly-q feathered) bred to a Silkie, to get Silkie style feathers that are curly-q. (They've actually refined the terms frizzle, sizzle, and what not, but I've forgotten what they have defined as what.)
If a breeder project culled her Sizzles that weren't sizzled, but mixed feathered, or simply sold Cochin-Silkie mixes, the owner could have heard bred from pure-bred "Cochin" and thought they had pure bred Cochins rather than Silkie-Cochin hybrids. She may be astonished she doesn't have true Cochins. (A quick google search would have rectified that).
But it is no excuse for selling misinformation. If you sell birds, you need to know what it is you've got. When a seller is clueless, misspells the breed, has non-standard features as "standard," you know you are dealing with a backyard breeder who is not paying attention.
When I sell mixed breed birds or hatching eggs (my olive eggers), I let people know that this isn't a breed but a hybrid that will NOT breed true. I show photos of my parent stock and photos of my egg colors.
I like my barnyard mixes as I breed for egg color. I've got some lovely ranges now. But I am very, very careful to not pass along misinformation. That is what a responsible breeder does.
Thankfully you have helped to educate and check this breeder so that they know others know they are passing along misinformation.
Look for a breeder who is proud of their parent stock, shows photos of the parent stock, and breeds to standard.
Educate your eye first by looking at good quality examples of the breed. Feathersite is a good place to research as well as Cackle Hatchery or Meyer Hatchery. Google the Standard of Perfection as well for your bird type.
Good luck finding some nice Cochins. Thankfully you caught this BEFORE the sale.
LofMc