The ethical question I do agree with you. KNOWINGLY SELLING SICK birds are a huge NO NO.
I think everyone here on this site have fought chicken illnesses. I know I have in my very short chicken career. And usually it is caused by introducing new birds into your flock. That is why I like and prefer to buy eggs. Unfortunately, there are times when, my only choice for me is to buy a living bird. I did so a while back, from a supposed quality exhibition breeder. And when I did I brought back everything that his birds ever had. I know that. Even IF they look healthy, they have things that mine have not been exposed to and theirs are carriers.
My new philosophy, is to be like the old time chicken farmers. I took my sick bird into my brooder room with my new born chicks. Some did die, most lived. Now all that are alive are very healthy and bulletproof from this disease. And have been threw the gaunlet and survived. And are laying eggs and having healthy chicks.
Today too many breeders try to isolate their flocks from disease by containing them in sterile conditions or shooting them up with injections. This is such a risk. Eventually your birds will confront this stuff. Then you REALLY can loose. Or those who you sold them too, might not put them in a sterile isolated pen. And then you have that reputation of selling sick birds, or birds who have little resistance.
I am not advocating selling sick birds. I am too tired of loosing them. By doing this, I find out what breeds are stronger and those that are weaker. Surprisingly, the high dollar, exhibition birds are doing rather well in my new system. I hatched out 2 dozen Orpingtons, and NONE have flopped over and croaked. My Marans are a different story. The Marans breeders by selecting ONLY for either their lines, are getting close to ruining their birds ability to fight off any disease.
Again, I do not sell birds that are sick. But, to say that I am not, or other breeders are not selling carriers of something is a misleading belief. We all like to think we are reputable. And each breeder has their opinion on how to protect their flocks. Mine is to not get to concerned over this. I no longer have sicks chicks, and birds. Their all rock solid. Good luck.