I've never had a problem with tickets purchased off of Ebay. I definitely agree that seller's feedback is the #1 thing I look at. Most of the ticket sellers I've purchased from had at least 50 positive transactions and no negative feedback. If you're worried about where they got the tickets from, then I'd email the seller and ask. Also, I highly recommend using PayPal cause if there is a problem, you're better protected. Finally, if you're really worried about the ticket, take it to a local ticket outlet and have them take a look. I purchased a pair of tix to one of the NJ Elevation shows and they were partially black (you could still read the copy). It freaked me out and I took it to TicketMaster where they confirmed that the tickets were real. Apparently heat and/or intense light, like sunlight can turn the tickets black (which is why they put them in an envelope when you purchase them).
TicketFast makes me nervous but I wouldn't select that option from ticketmaster either. I'm old fashioned that way!
Good luck!