Paypal also offers people the ability to scam you when you are a seller.
People can raise issues with the goods such as they havent received them or the item is not as described and there are very few checks that paypal do before issuing a charge back - three of these can be raised in a year.
Edit: Hopefully paypal have closed the loophole on checking if the good have been signed for, as pointed out by DM.
Really they need to look at a way to police paypal effectively from both a buyer and seller point of view.
Edit: The 21 day rule - pointed out below will hopefully deter some people too.