That's where collective bargaining comes in. supergreedymegacorp inc only has the massive bargaining power because of its size. with collective bargaining, you take away that size advantage. Without the distortions placed on the market by state regulations, the two groups could come to an amicable arrangement, that benefited both parties.
Yes, but currently there are laws (like Taft-Hartley and even the Wagner Act) as well as other laws like "right to work" agreements etc that greatly limit collective bargaining power. Though we may have different interpretations of how we think a voluntarist society will eventually evolve, surely you will agree with me that among the capital-L Libertarian movement in the US there tends to be much more handwringing over things like minimum wage laws and safety-net programs that provide some kind of "soft-edge" to the predatoriness of state/corporate capitalism than many of the regulations that give corporations that power to exploit
Here is a very good analysis of what labor struggle in a free market could look like, combined with a critique of the labor analysis the Libertarian Right often uses
http://c4ss.org/wp-content/uploads/2010/09/C4SS-Labor.pdf