I have a scanner, and find things on clearance, that sell for full price on amazon. I also buy a lot of textbooks and resell them on there.
Example, Found some of those Wii Fit Boards for $20, that current selling new for $100. Found a USB GPS Data tracker for $15 that sells for $130. Those where my bigger finds, most times I'm say spending $5 bucks, sell for $25, enough of those finds start to add up.