I agree that the characterisation of labour-power that is crucial to Marx's
conceptualisation of capitalism includes that it be traded as if it were a
commodity. It is not just the general human capacity for mental and manual work,
but the specifically capitalist form of that, under which it is grasped by the
value-form as potential wage labour and the capacity to create not just useful
objects, but commodities in which the trans-historical category 'useful object'
appears now as use-value inextricably wedded to value in the commodity.
BTW (I've been dying to do that ...), the characterisation of labour-power as a
quasi-commodity facilitates the grasp of a key irresolvable conflict (=
dialectical contradiction) of capitalism: the endless struggle to commodify
labour power - in which the working class seems to be suffering terrible defeats
in the last 15 years of liberalisation of labour markets, and rolling back of
welfare states.
Comradely greetings
Michael W.