Purchasing sweat shop products - quandary?

Ok, so i'm going to pose the question - If people stop buying sweatshop made stuff, what then happens to those people who rely on that (meagre) income. Are we such utopians that we think that by boycotting, it will lead to better conditions? And another question - if you buy something that was originally sweatshop made (eg Nikes) , but purchased them second hand, does that negate any impact?