Any chicks you sell would have a chance of being carriers unless those eggs are hatched elsewhere. I would be horrified to buy chicks from someone with a Mareks positive flock, no matter how much you try to keep them from exposure. Mareks is spread in feather dander, and you know that stuff gets...
I would stop trying to raise chickens to sell. Your birds even without symptoms will infect other flocks if sold or rehomed. You have Mareks in your environment, and it could last for months to years after the last chicken is gone. They all have been exposed. It is wonderful that no others are...