"If the two subspecifics are so radically different that actual science should award them subspecific status, then why would you have trouble telling what you're puportedly going to buy?"
It's because they've been crossed together. You could take a bullsnake and a corn hybrid, then breed the offspring back to a corn or a bull, and then breed those offspring together, and then mix it up a whole lot more. Just like what has been done with diamond and jungles. Pretty soon there would be some hybrids that would look like pure corns or pure bulls.