To make it simple, refer to theymos post here:
Money sent through PayPal and any other money transfer service that accepts credit cards can be easily reversed by the sender, even after several weeks. Therefore, you should only accept PayPal in trades with people you trust very highly.
This is why newbies are not likely to sell their PayPal USD for bitcoins successfully.
-snip-
As you are a newbie people will not trust to trade their btc for your paypal because paypal has a dispute option where you can reverse a payment.
Afaik, it is also not safe to buy and sell bitcoin using paypal because some users I've seen had some complication where paypal asked proofs of what is sold and they can't give anything because it is bitcoin.