At a Sunday meeting, Napoleon announces that the Animal Farm will engage in trade. He says this is not for commercial purposes (i.e., profit) but so that the farm can obtain badly needed supplies, especially for the windmill. He asserts that the needs of the windmill must override other considerations. Therefore, he has authorized the selling of a part of the wheat crop and a stack of hay to raise money, and warns the hens that their eggs may need to be sold as well.
All of this makes the animals uneasy, as one of the tenets of Animal Farm, articulated initially by Old Major and adopted after Farmer Jones's overthrow, was that the animals not engage in trade or money making. That was what men did, and the ways of men were evil.
Napoleon, of course, does not have the best interests of the animals in mind, and this is all a ruse to raise funds so that he and the other pigs can live comfortably and even luxuriously on the hard labor of the other animals, replicating the exploitation of the human world.
This is a good question. The whole story is about manipulation and gaining power and keeping it.
At the start of the book, the revolution sounds like a great idea to bring about freedom from an unjust situation. However, as the story progresses, it becomes clearer that those in charge are sincere. For example, the pigs steal the milk and apples, and they justify it with fear tactics - "You don't want Mr. Jones to come back, right?" Later on, they make seven commandments, which should govern all animals, but the pigs change the commandments when it serves them. The worst situation is when the pigs use Boxer, and when he cannot work, they sell him to knackers to be made into glue.
The point is that Napoleon does whatever it takes to gain power, wealth, and prestige. So, if he could trade with other farms, why not? He does not really care about the other animals. This is why at the end of the novel, he turns into a man.