Opinion warning!
Even though you can keep them for weeks at a time, don't -- at least not to sell. If people are going to be paying you for eggs, they should get fresh ones. If I went to a farm to buy eggs and someone handed me a carton with 6 week old -- perfectly safe, perfectly edible -- eggs, I'd be more than a little ticked and wouldn't come back.
...
That aside, Mother Earth News did an experiment back in the 70s and kept them in the 'fridge for 9 months and they were still edible. Watery and not so tasty, but edible and safe.