We had to use the paypal dispute once. My girlfriend bought some shoes that was not the size advertised. Seller did not respond to any emails, etc. Finally, when paypal send the seller the final warning, the seller responded (death in family, hence no communication, although they've been busy listing and selling other items).
As per paypals instructions, we return the item to the seller, via recorded mail. We email the confirmation code to paypal, who then refuses to pay, because the initial of the person who signed for it, because the name of the seller did not match the name of the person signing for the item. There was a 1 letter difference, which was obviously a typo from the Royal Mail OCR software/person.
I send off a complaint. 3 months later, I get an email from paypal, asking what it was I was complaining about, because they can't find anything in their records. I didn't even bother replying.
Convenient, yes, trustworthy, no.