I sold fertile eggs 3 weeks ago, I also but 10 egg in my incubator and 9 chicks hatch, but the person that got fertile eggs from me shortly after, just contacted me and said non of the eggs I sold him hatched.
he had told me before that he had got 20 eggs from another person and only 2 chick hatch, so I offerd to hatch some chicks for him in my incubator because his incubator is not werrry good :/
so it could also be that the person just didn't have a good incubator, I have one really bad incubator that showed me that the heat in the incubator was fine, but I felt like sometimes the eggs where cold so I took another thermometer and then I saw the incubator was somtimes to hot and some times to cold so I stoped use that that incubator and bought a new one which has been much better, 9 egg out of 10 hatching is really good in my opinion so I'm really happy with that new incubator I got
and also it's always a risk ordering eggs online to so it's could me many reasons why the eggs didn't hatch :/