If you seriously think China is communist these days, you haven't heard of Deng Xioping's maxim, "To get rich is glorious". Secondly, as for the slavery - you have totally failed to prove a direct link between free trade and slavery. In fact, let's put it this way - if countries cannot participate in the world trading system, are marginalised and poor, they are more likely to have slavery or extremist governments: countries with outright slavery like Niger and Mauritania have no economy and little trade to speak of. Countries with truly operating communism like North Korea have policies you would like - Kim Il-Sung committed them to a policy called "juche", which means "self-reliance". They shut out the world. And look where it got them.
Ivan