The best thing to do is identify what killed your youngsters and other dragons. For every dragon you sold since buying the unquarentted dragon, do your best to get ahold of them, explain what happened and the disease that infected your stock when you bought the new dragon, to warn them to watch for symptoms or to just get a vet inspection to see if the animal they bought from you did get contaminated. You can offer to re-imburse them for the cost of the dragon, take it back, and may some compensation for vet/treatment.
You made a mistake and have already paid for it due to the lost of your animals, letting those you sold to aware of the possible danger (and if they didn't make the same mistake and kept the new addition in quarentee, greatly limit if not completely eliminate contaminating their existing animals) so they can take appropriate action or decide to return the dragon to you so you can make sure it indeed has a clean bill of health.
If they didn't practice quarentine, and their existing animals have been infected, deffinitely re-emburse the cost of the dragon you sold them, and maybe pay $100 for vet bills etc. I don't think you should be obligate to pay replacement value of th eir stock etc...because it isn't your fault they didn't keep the new addition under quarentee, which all new animals should be kept under, no matter how reliable the breeder is, there is always something that could happen (shipping stress could elevate parasite levels, the dragon may come in contact with an airborne virus, if say, other dragons from other sources just happen to be on the same flight, or have been in the past and the contaminant didn't disperse, etc.) However, if you feel you should and are able to, you certainly can offer to pay more to cover vet expenses, if the new owner has to get all his animals checked and/or treated for the disease you had problems with.
Moving as fast as possible to notify those who bought animals from you d uring the period you had the new dragon, is the best thing you can do, to give the new owners choices and to know to product their other animals. It's just like when you buy a car and 6 months later, the manufacturer realizes there is some sort of defect, immediately they take steps to notify owners of that particular vehicle model/year has the defect and owners should take their car directly to their dealer, and have it replaced, most often completely free of charge.
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PHLdyPayne