I don't know if anyone has posted this yet, but I found a foolproof guide on tf2pricecheck:
http://www.tf2pricecheck.net/thread-3246.htmlIf you're trading things of value (bill's, buds, unusuals, max heads, etc) it is
imperative that you check an item's history before you sign off on a trade. Duped items are significantly cheaper than original ones, not to mention very hard to sell considering no one wants dupes.