Democritus (c460-371 BC) of Abdera was not post-S, although Lucretius, yes. But Marx's work is really about the Atomistic philosophy, focuses on _ataraxia_ as a principle of understanding the world and really dwells on Epicurus' (341-270 BC) writings on the subject. I think Marx was using ataraxia as a hammer to break through Hegel's concept of philosophy as spirit and thereby pointing to some kind of theistic argument. He focuses especially on Epicurus' denunciation of belief in the gods as a fear of mortality.
But I read Marx's work years ago. I did do a paper on it.
chuck miller http://www.users.uswest.net/~bautiste/index.htm