Sadly, if a bird with MG had access to your flock, even if none of your birds have symptoms or appear sick, they are all carriers now. This is a classic example of why the disease is so common. Vet statistics report that between 75 and 90 percent of ALL flocks in America carry MG, yet most people have no idea. You would have no clue that your birds were infected until the flock encountered some stress, like the introduction of new birds, molt, extreme temps, etc, and even then, if you have otherwise healthy, vigorous birds with quality immune systems, they are more than able to keep most bacteria in check.
I find it particularly sad that so many people try to stigmatize and demonize others who find out they sold infected birds, when the reality of the situation is that 8-9 out of 10 birds in the country are probably carrying and their people are simply ignorant of it.
I am beginning to feel the more I read about people and their experiences with these diseases in their small flocks, that the best way to go for small breeders is to stop focusing so hard on feather patterns and recessive genes, but put some serious emphasis on hardiness and vigor so that more birds have good immune systems and are able to beat disease. Anyhow... just a thought.