There are quite different notions of what the term "socialism" means, so what follows is based just on the one I work with. What socialism means is that the goals of trade are defined radically differently than what obtains today. In today's capitalism, trade is first and foremost about private profit opportunities. You trade if you are a business and see profits from doing so. The goal is private profit and collateral costs (externalities) are not the concern of the trader, no matter how huge or negative they might be. In socialism, the framework of calculating costs and benefits of trade are different. The social externalities are estimated and counted into determining whether a social profit is or is not entailed in any proposed trade. Moreover a socialist perspective would add goals not relevant in capitalism. For example, to correct capitalism's uneven development (the last 300 years of dividing countries into rich and poor) a socialist trade policy would stress offsetting key imbalances in the interests of equality, world peace and so on.