Both of these can be negotiated freely, so both the worker and the seller are willing to do the trade.
And yet, there is no negotiation between the worker who creates the product and the capitalist. That is root of the problem, as capitalist is the only one that makes all decisions.
Wrong. I as a worker can demand/ask for a higher salary. If I do not get it I can go to a competitor. There is a marketplace of supply/demand out there.