Economist's View: 'Markets Erode Moral Values'
Apparently some German scientists have proven this with some carefully-designed behavioural experiments. The comments in the link above also mention some more obvious points: you would probably never buy anything from the shop of a man you knew to be a child abuser. But you are probably wearing clothes stitched by an asian child slave.
Perhaps also related to the following phenomenon: What Is Diffusion of Responsibility