Simply put: you can buy stuff from Auctions and sell them.
Especially computers, laptops, monitors etc.
you could sell them at your neighborhood flea market if there is one.
Try auctions likehttp://www.govdeals.com/index.cfmhttp://www.liquidation.com/
(moreover, you can also buy used supplies from the govdeals.com website, like printers, and industrial products etc)
*a note about govdeals.... you have to pick up the items from their locations, so buy locally or near enough that you can transport them