I think you need to let your buyers know that AFB has been found in colonies that originated as nucs from your yard, and that there is the remote possibility of it being in the nucs they purchased. (Presuming you believe this to be true) It would be fair to offer to take the bees back and give refunds. Encourage people who wouldn't necessarily know AFB to have the bees examined by someone with an experienced eye. I would take the further steps of reiterating that the bees have not been treated with anything to suppress AFB, and that as far as you know the nuc(s) they received are free from AFB. State a deadline for taking back bees / offering refunds.