If the item being sold is a physical item, you're protected as long as you leave a return address and can provide tracking information in the case of a payment reversal. If you're selling non-tangible goods like services or digital goods, there isn't a sure-fire way of protecting yourself. It really depends on the situation; if it's a domain sale, then you're more likely to win the Paypal dispute as your registrar will have records of your ownership plus you probably have some sort of agreement or contract made out with the other party. From my own experience, Paypal does not do much to protect sellers of digital goods and services.