Ok, we take no responsibility for what goes on here - if you're buying or selling then use common sense before you part with your readies. Try to suss out if someone is geniune before doing a trade.
And don't publish your phone number/address or when you're going to be out of the house and where the spare keys are stashed, etc.
Use the private messaging system / email and keep records of all corrospondence, and consider using an Escrow service if you're parting with a lot of money.