I was perhaps being a bit glib in referring to axioms in Marx; what
I meant was that the propositions of the labor theory of value were,
Marx believed, derivable from the concept of the commodity, in
the "contradiction between exchange-value and use-value" (TSV II,
p. 501 Progress press).
I have developed my own set of axioms from these, as a codification
of Marx's dialectic as I interpret it; but I would have to agree
that Marx made no such formal statement himself.
However, the point remains that Marx did believe that he had
derived the main claim of the labor theory of value--that labor
is the only source of surplus--from his analysis of the commodity.
Cheers,
Steve Keen