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)