No, they're flagged because Craigslist doesn't permit the sale of animals through its site. It only allows adoptions.
Obviously, if someone has a ball python listed for $250, it is for sale, not for adoption. They aren't all stupid.

I honestly have no sympathy for the sellers who get flagged off. They are breaking the TOS, and frankly, Craigslist should simply ban them after the third time it happens.