Ask your customer what she considers an "old" egg and what she considers "fresh." If she wants them straight from the chicken, charge her extra!A month seems a bit old to me... I would be dipping/sanitizing them before they go in the fridge if you routinely keep them that long before selling.
A month seems old to me, too, but I've never really studied shelf-life. I don't sell eating-eggs, but I will occasionally "share" extras less than two weeks old. Anything older than that generally gets fed to the animals.