I believe PayPal is the safest way to trade, BUT I discovered the hard way that some buyers can use it in a fraudulent way. Here is what happens and how to avoid getting caught.
Someone buys an item from you and pays by PayPal. You check your PayPal account and see the money is indeed there, so you go and post the item. A short time later you get an email from PayPal saying that the payment has been withdrawn. No ammount of correspondence and pleading will get your money (or your product) back. So now you've lost your product as well as the payment. How did this happen?
Well, the buyer, soon after paying you and receiving the "item posted" email from eBay, contacts PayPal and makes a false claim against you. PayPal immediately stops/withdraws the payment, and there isn't much you can do to prove your innocence. How to avoid this?
Don't just check your PayPal account! Transfer the money immediately to your bank account and wait until it clears BEFORE you post anything. This applies to all types of payment. Do not post anything until the money is in the bank.