I think you're missing a larger point here.
Let's review, for a moment, one of Marxism's greatest maxims. In so many words, what causes conflict in society is the lure of, and the distinction afforded by, material wealth. Particularly surplus material wealth. Eliminate surplus material wealth and distribute the remainder equally, and societies ill's would be cured, according to Marx, since most of societies ill's were caused by that old bugaboo; human nature, espeiclaly the more venal aspcets such as greed, jealousy and gluttony.
With the advent of the machine ecomony in Marx's day, the means existed, for the first time in human history, to produce enough goods, at affordable prices, for all of society to live at the same level of existance, and therefore there was no longer a need to indulge in distinctions based upon material goods (wealth)and the problems such distinctions posed to society. Provided, of course, there were well-intentioned Marxists around to decide how the goods got distributed. Marxists, you see, were not supposed to be subject to the whims of human nature.
Taken hand-in-hand with the newer theories of the planned economy coming into vogue at that time, and Marx kinda-sorta makes sense in a dispassionate,clinical way.
The problem with this theory is that a) the machine economy produced surplusses that it often was impossible NOT to distribute and b) human nature never actually goes away just because you conveniently ignore it. What Marx was getting at was a simple exchange of suprlusses in order to hold that which could not be controlled (human nature) in check.
For example, if Nation X is a fully-industrialized society and the bordering Nation Y is an agrarian one, then there is bound to be friction. Nation X has, in it's industrial potential, the ability to militarily and economically dominate Nation Y. It's citizens have a higher standard of living which incubates jealousy within it's neighbor's population. Nation Y, on the other hand, has to contend with a richer, militarily stronger neighbor which is a magnet to it's disaffected masses. The solution is Free Trade, in the sense that X trades it's surplus industrial goods for Y's surplus agricultural products. It is the simple exchange of goods with the idea that both sides should be satisfied in the name of parity.
Free trade, in the literal sense, extends beyond the mere exchange of goods. It also incorporates the exchange of labor, capital and ideas. None of these is necesarily amicable to the ideals of Marxism or to the idea of a planned economy.
Marx, therefore, is not a "Free Trader". He is simply someone who advocated the exchange of surplus wealth, where applicable, in order to prop up his fantasies of international brotherhood.
In the example cited above, there is nothing keeping Nation Y from matching the industrial output and stregth of it's neighbor X, except the notion that the citizens of Y should be content to suck off of X's teat in a symbiotic relationship. The overriding concerns in this example is not national greatness, equality or greed, it's supposed to be simple peace. In this regard, Y, in the name of peace, is to simply forego any attempt to equal the industrial capacity of X.
That is hardly "Free Trade" is it?