The fcf for retailers can simply come from paying suppliers later than getting paid by customers. While a cheap source of capital (even this is doubtful), not exactly a winning strategy in the long run. The fcf will stop once revenue growth slows. As capital it is only cheap in the sense that you don't have to constantly raise equity for growth. Without earnings your equity base can't grow w/o getting more from shareholders.
It is if it is stable enough (that is the negative working capital). In other settings (such as insurance) it is also known as float. Negative working capital is great if you can get it and keep it. The problem with the associated fcf is that the fcf is the derivative, literally. So if the working capital stops going more negative the associated fcf goes to zero.
