Mr. Rousseau was a dealer, a middleman. He was a man who bought and sold borzoi for a profit, often to Americans. We can perhaps have in mind the following:
"There were a lot of dogs in Perchino, new ones were born every year. Regularly compiled a list of "defective" dogs and these dogs were sold or donated." .
One can wonder how many of these "in the eye of the Russian breeders, "defective", borzois passed through Mr. Rousseau's hands!