John is a human cow, a person kept in a barn or shed and regularly pumped for milk. He's abused because his breasts are small and don't produce much milk, but then his keeper is mysteriously murdered. John saw something that could help solve the case, but no one thinks to ask because human cows are almost as dumb as regular cows, right?
Sherlock figures out that John knows something, but the only way he can gain access to him is to buy him. Sherlock doesn't quite anticipate what it takes to care for a human cow, feeding him, milking him, and all that. nor does he expect John's insistence at caring for him in return.