pricecharting.com

It's a really helpful website for checking prices of games and consoles. For example, searching "Black Ops II Xbox 360" will give you helpful information on the current prices, price history, listings, and more of the game. Let's say your searching through some online marketplace and see a used PSP being sold for $140, well you can go and look at the selling price on Price Charting and see it's $25 more than the average selling price. It's not really anything that big but it's a helpful enough tool I thought it'd be cool to share.