Safety

Do not extend payment to anyone you have not met in person.
Beware offers involving shipping - deal with locals you can meet in person.
Never wire funds (e.g. Western Union) - anyone who asks you to is a scammer.
Don't accept cashier/certified checks or money orders - banks cash fakes, then hold you responsible.
Transactions are between users only, no third party provides a "guarantee".
Never give out financial info (bank account, social security, paypal account, etc).
Do not rent or purchase sight-unseen—that amazing "deal" may not exist.
Refuse background/credit checks until you have met landlord/employer in person.