The Marxist's argument goes as follows:

All things are produced by the workers, either directly by the workers working or indirectly by the workers building machines called capital, which then help produce goods. The capitalist consumes some of it, therefore the capitalist exploits the workers.

Now if you think of it, you will notice that there is a hidden libertarian premiss in there somewhere. Namely that things belong to the people who make them. If the socialist really believe that the right premiss was - from each according to his ability, to each according to his needs, it would follow that the worker is good in working and the capitalists need Rolls Royces, and that there would be absolutely nothing wrong with a capitalist receiving profits.

David Friedman "Ethics vs. Economics"