Theres a few ways
If you dont know the odds and ends of paypal its easy to get tricked

1. theres something called a e-money transfer (or i forgot what it is) but it is like a cheque where it looks like they have sent you the money but infact it takes a few days to clear the funds. They could pretend that they sent you the money (because it looks like that but it is actually being cleared by paypal) and then they can withdraw these funds before it is being cleared

2. With this scam they usually will send you a request for money (make up some story about having an issue with the courier and they require x amount of money first which he will later send back to you) then you accept the request for money and thats the end of your money!

Google the scam in google and you will see the different ways people have been rorted.