>It appears you would reject the premise of running a co-op in a capitalist society because it does not start from a fundamentally pure starting position

No, not at all. In fact, I support local co-ops. My (and Marx's) only contention is that it is not sufficient to call this "socialism" or "socialism within capitalism", or "a socialist part of capitalism" because socialism is by definition the ending of the specific historical production of commodities and class society. A co-op is an admirable thing to run, but it's not "socialism".