This website helps you identify unexperienced Team Fortress 2 players with valuable items. Trading with these people will often result in better deals than you would get normally. All you have to do is follow these steps:
Click a victim's name to visit their profile, and clicking an item shows you his/her entire backpack.
Note that hats are always sorted before weapons (a Unique quality hat will always appear before a Vintage weapon, for example), and Unusual hats are considered the best quality.