the wording that i just got in an email selling something and using paypal as a gift:
A RANDOM just sent you money with PayPal.
Reversals:
Please be aware that your payment can still be reversed, (e.g. if it is subject to a chargeback), even after you have posted the item to your buyer. Complying with PayPal's protection programmes and following the trading guidelines, in our Security Centre helps to protect you from things like chargebacks.
so there is some easy way for a buyer to reverse it. easy enough that you need to be warned to protect yourself from chargeback