Paypal would cover you for that, just make sure you go to someone with a good rating. I have bought a few tickets on ebay, sometimes cheaper than originally cost but thats been a rarer occurance.
This year the Reading Festival organisers used viagogo. Manchester United FC also uses them - it is about as 'offical' a place as you are going to get - its basically for people who can no longer go. The Scarlet Mist website also allows people to sell at face value only for gigs etc.