[thorough]

Hypothetical question:

I have little money but I write an amazing book and it sells to millions. Each books takes, say, $5 to print, and yet I sell it for $15. Therefore I end up wealthy from this endeavor.

Did I exploit the people who chose to buy my book? The people who printed it?

[666lumberjack]

Did you personally sell millions of books on your own? Probably you used some service like amazon in order to sell those books and fulfil the orders, in this hypothetical, in which case the lefty argument would be that you are indirectly profiting from the exploitation of both amazon's workers and the employees of the printing company.