This was part of the puzzle that is also the part that makes it impossible for communism to really get traction in an industrial society.
Those unskilled laborers that are so exploited aren't all stupid; many can and do move up in the company. Sure if the guy can only run the machine he's just another part and replaceable but if he can maintain the machine you give him some tools and a raise and get another "part" to replace him on the line. If he can teach others to maintain machines... another raise and he's middle mgt. If, perhaps, the individual is smart and creative about what it takes to get the material flow going or the packages shipped better / faster, a raise and promotion are in order. Hard to sell communism when a mechanism for the worker to advance is built right into the capitalist system. This is the rise of a technical and managerial class.
As for your question "who buys the products?" It starts out that they are exports and for the very rich but if you follow the scenario above... at some point more money and a title aren't getting it for these upwardly mobile types... they need time off. What we see here is the birth of the consumer society; these folks have disposable income (surplus after the basics of feeding, clothing, and housing their family)... what they needed was time off to spend that money on the products they produced. Hence shorter work weeks and later shorter work days. Hard to swallow?
Consider Henry Ford (much later) kept the union at bay for a long time by paying his people more that "scale", in fact, enough that
the average worker could afford a Ford.

This is a quick and dirty explaination, I admit, but indeed the basis of a lecture on how we get to be a consumer society. The next day would be the rational of the labor union.