You're looking at something called a "chargeback"; when a customer, legitimately or illegitimately, disputes a charge on their credit card. This can cause problems for merchants, as most laws are designed to protect the consumer rather than the merchant.
PayPal has an FAQ called
Understanding Chargebacks which will probably have the answers you're looking for.
There's a service called
Chargeback Guardian that might be worth investigating. I'm not sure how big a threat chargebacks are to a merchant, and their site might be alarmist, but it has more details about what can go wrong.
I hope this helps.
