i can speak first hand here, as i am breaking down my 360 gallon. my tank has flatworms...even after treating it twice, it still has flat worms. i have made all buyers aware of the situation and i have been setting up 5 gallons of water treated with FWE and dipping everything i sell for 20 mins as the buyer stands and watches. i have no idea how i got them, but i do now want to pass them on to others!
people who knowingly sell infected corals are just jackasses...although, ill bet all of us can go to our LFS and see flatworms, aptaisa, valonia, etc. infected corals and live rock. my LFS didnt even know red bugs existed until i pointed them out on a few colonies they were selling.