The reason that Napoleon wants the eggs to be sold is, of course, for money. What Squealer announces is that the money will be used to buy grain and such to tide the animals over until it gets to be summer and they can get more food.
When the hens hear this, they protest and start to smash their eggs. They end up getting killed. They are starved to death. Napoleon orders that anyone who gives them any food will be killed. The dogs ensure that no one does help the hens and nine of them die.
It is important to note why Napoleon says the hens' eggs should be sold. He claims that the hens general make-up almost makes it impossible for them to contribute to the labor and construction of the windmill. But, they do have a commodity others value, and that could be their contribution: their eggs.
The hens stage a rebellion of their own which includes laying their eggs from the rafters so they break on the ground and are of no use. Napoleon responds by withholding their food. After five days, a few die. The rest decide its not worth it and give in to Napoleon's demands.
In Chapter 6, Napoleon announces that the animals will begin to engage in trade with their neighbors in order to obtain necessary materials that they are unable to produce on the farm. In Chapter 7, Napoleon accepts a contract through Whymper for four hundred eggs per week. The price of the eggs will help pay for enough grain and meal to keep the farm going until summer. The hens are outraged when they hear that their eggs will be stolen and stage a rebellion. The hens begin to fly to the top of the rafters to lay their eggs. They then knock the eggs off of the rafters and let them fall to the floor. Napoleon immediately orders the hens' food rations to be stopped and threatens to kill any animal who gives the hens food. The hens hold out for five days before they finally capitulate and return to their nesting boxes.