A marxist interpretation says that labour market is not like any other market because the supply and the demand have significant difference here: A worker sells her labour but gets paid only for her "humanity". Marxists argue that
value of labour > value of reproducing human
and this is where surplus value is derived from. Therefore exploitative capitalists don't pay for the full value of workers' labour.

It should be clear that as Noung writes not all people possess the skills needed and therefore validity of marxist theory is diminished. But I would say that it still has some relevance on the spheres of production in which hardly any special skills nor education is needed.