Argue that free trade is not fair.
To argue that free trade is not fair, you should point out that not all countries are able to compete with one another on anything like an equal basis. This might lead some countries to get stuck doing the unskilled work for the rest of the world. If a country has no real advantages other than cheap labor, it is, one can argue, hard for it to get ahead through free trade. Companies from rich countries can come in and exploit its cheap labor without doing anything that will actually help the country’s economy grow. From this point of view, free trade benefits those countries that are already rich.
"Free trade" is situated in a context of great material inequality. Therefore, the contract between nations is exploitative. In a word, the rich get richer at the expense and to the deteriment of the poor.