1. Scammer - Malicious BNET user with the intent to steal things of value or importance.
2. Scammers will make up various stories, all in an attempt to get at your items or password.
3. Don't interact with them. If anyone asks for your password - anyone - don't hesitate to squelch them. If someone tells you they will trade you their item for your item, but you must give them your item before you recieve theirs - don't hesitate to squelch them or leave the game.
Use common sense, don't hand your password out to anyone, don't give away items unless you are getting a fair trade.