i've had a lot of experience selling e-tickets to events and concerts over the years on ebay. i've always used paypal and i've never had any problems - that being said though, it's definitely open to abuse.
most of the time, people want to do the right thing and get tickets to an event that they couldn't get any other way. they're often willing to pay larger amounts of money for them (as you're probably aware).
the grey area is that e-tickets need to be emailed to the person - which means you aren't meeting the seller protection obligations by sending them via a tracked service to the address in the paypal checkout. however, the buyer will be covered.
once you send the tickets in .pdf to the buyer - if they want a refund or open any type of case which is found in their favour - you can no longer sell the tickets to anyone else - because the buyer already has a copy of them and can gain access to the event.
the other thing is - there is no way of telling whether or not the person gained entry to the event; therefore, if they wanted to - they could go to the event, gain entry and claim the tickets never worked and you'd have a rough time trying to get any ticketing vendor to verify whether or not the tickets were used for entry.
also, it will always be their word against yours if they say they couldn't gain entry - they could say you sold multiple copies of the tickets and there's no way of refuting it.
there are better avenues to sell digital tickets - i would only ever use ebay as a last resort now. there's many resale sites from the ticket vendors themselves or others which you can find very easily.
just reiterating though - i've sold hundreds of tickets and have not once had a problem - but should you take a hit, chances are it will be costly as the tickets being resold are generally in a high price range :] don't know if that info helps.
