Well, I am one of those 
Let me explain what I thought and what I think. It may help you to figure out how to argue with marxists and convert them.
CONFESSIONS OF A MARXIST MIND TURNED IN FAVOR OF FREE MARKET CAPITALISM
I used to believe that people’s motivation was more or less constant and not so sensitive to incentives. Thus people would strife and innovate independently of what economic system there was. This was never made explicit in my brain, but it was nevertheless an underlying assumption. A necessary corollary of this assumption is that differences in wealth has to derive from a combination of the following causes:
-
differences in innate abilities
-
exploitation by some of others
It was clear to me that differences in abilities could not explain the differences in wealth that I saw around me. After all, no Robinson Crouse could end up super rich. That could only happen by taking advantage of others. (And, as I thought, necessarily at their cost.) Under this assumption, it was natural to think that society collectively should gather to protect itself from the exploiters, just as libertarians usually think that society should collectively defend itself from outside invaders through national defense. The taxes that capitalists had to pay was not “their” money in any meaningful way, since it was basically stolen from the workers. Therefore the state should take it all back and use it for the common good. In fact, I thought that the government should not only take back what the capitalists had stolen from their workers, they should take the further step to artificially erase the innate differencies in productivity. The slogan “from each according to his ability, to each according to his need” should be the guiding rule.
Why did I change my mind?
I need not to go into detail about the merits of free market capitalism, as you all know it. Basically I realized that people react to incentives and that this was the underlying reason why capitalist nations were richer than communist nations. I also began to understand how free markets work in allocating resources, and why it was futile to believe that state planning could do the same job. I also realized that one cannot count on the government to act for the common good, since state bureaucrats are not necessarily more angel-like than businessmen. I now realize that the “natural selection” that acts on business is essential to drive out inefficiency.
In short, I have changed my underlying assumptions about how society works and come to different conclusions. This was not done by divine inspiration. It was done by starting to read about economics. I did it reluctantly. I thought that economists were not interested in understanding how the economy worked and had no theories about it. I thought they only invented theories about how to earn money by not working (i.e. “robbing” from others). When I started to learn about the workings of the market I was really thrilled. It was the beginning of a 3 years battle where old “obvious truths” were to battle agains reason and statistics. There are literally billions of neurons that have to connect in a different way when you change your world view in such profound ways. Fortunately I am a man of science, so I know that empirical data has to be accepted and interpreted rather than explained away. I think this helped a lot. But more than anything else, I benefitted from reading Sowell. It was like reading the Devil’s poetry. So clean, so logical, and so contrary to anything I had always thought. It is a strange feeling to have your assumptions and convictions smashed down mercilessly without being able to get upset. Sowell kills almost any argument against him. One can only ignore him or respect him. But to ignore would be intellectual dishonesty.
To sum up, I should perhaps say that I am probably not a “true” libertarian, as far as I understand the concept. By that I mean that I do not warship freedom. I think freedom is nice, of course, but not sufficient as an argument for the free market. Under my old assumptions and conclusions, i.e. that “freedom” led the masses into misery, I condemned freedom since I saw it as being beneficial only for a small randomly selected group. Only under my newly acquired hypothesis that freedom is the most efficient method to bring prosperity to the masses do I embrace it.
I therefor strongly suggest that, when arguing with marxists, do not emphasize freedom. Do not talk about profits in terms of rewards for good work. Talk about it as a necessary evil that has to be there to get those small-minded people out there into being useful. If you have to mention freedom, treat it as a positive side effect, not as a goal in itself. Marxists think that this freedom is only theoretical, that in practice it leads to LESS freedom for the masses since it permits them to be exploited by the capitalists.
Good luck,
Martin