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.
Some other services that are not totally safe:
- Bank transfers (ACH, etc.) except wire transfers
- Most gift cards.
- Moneypak
- AlertPay
- Paysafecards
- Dwolla
- Western Union (they reportedly will sue the recipient to recover money in some cases)
Good to know about PayPal. I have an unpleasant experience with Western Union: I put a deposit via WU, somehow recipient was able to take money without my consent using fishing website, and the company refused to reimburse me. But not only that, in order to apply for reimbursement I needed to have written document proving that I have transferred money and that alone took more than a month. They were not going to investigate anything. Be it my fault or unlucky coincidence, WU is not that safe as they claim to be.