I don't think I would sell sick birds, just not moral. Unless you tell the people and their use for the birds is not affected by the birds' illness (whatever that is).
My big coop had a lot of deaths out of the blue. I have no clue why. These never got sick but I go from one coop to the other.
Waste not, want not!
She can always pay more of her own volition, or give you a tip
An educated population is a good population. I'd sure hate to go back to the days when only rich kids got an education. There aren't many jobs anymore for illiterate people.
The cheapest breeds from the big hatcheries straight run are around $2.75 and then there is the shipping

You can base your price on that. Local birds have no 2-3 day travel stress, no shipping cost.
Treats can bribe most any chicken. I have some that are skittish and sure wouldn't eat BOSS or scratch from my hand but they come running with the rest when treats are to be had. They just don't get as close. If any of my birds, even those that aren't skittish, feel they are trapped and start to freak, I just stop moving. Then they appear to think "Oh, no problem here", calm down and walk where they want to go.