Do you see this as a real potencial problem? Has already happened a fraud due this ?
Yes. The percentage of problems is actually very small compared to the millions of user on paypal, but protect your accounts in every way possible. Check paypalsucks.com for bad stories. Many stories are about buyers who pay and receive your goods, then complain to paypal and get a refund even though you dispute the claim. Perhaps paypal has improved this problem more recently, I havn't been keeping up.
Also check your PM. I sent more details about how I protect my accounts that may help you. One big change in paypal recently is in their own protection policy. Never ship to an address that is not part of the buyer's paypal account. If you ship to a different address, paypal will not cover you in case of buyer fraud. Always question and resist shipping to a different address than paypal shows for your own protection unless you're positive you know why and you're willing to take the risk.
I'm concerned now
That is a good thing. It keeps you safer :-)
Plaz