Well if you order it through Ticketmaster I really, really hope there's no way the same ticket got sold twice. With a site like StubHub they require the seller's credit card so that if there's any sort of problem with the ticket being fake, etc. then the purchaser is refunded using the seller's card. The reason why I chose not to do that is I found on a consumer fraud site (
http://www.consumeraffairs.com/entertainment/stubhub.html) that the buyer theoretically could go to a show, get in with no problem, but call StubHub after the fact and say the ticket was fake and they seem to err on the side of the buyer- although some of those stories also seem to contradict that, so who knows. Either way, the site seems fishy and I'd rather not deal with it.
As Tom stated, if you purchase online using a site like PayPal they resolve disputes similar to Ebay or Amazon so if someone sold fake tickets through that they would probably get refunded pretty quickly.
I kind of went on a monologue, but the point is, if you're going to resell a ticket over and over it's a bad idea to do it through a website like the ones I just listed above because you'll probably get caught and be on the hook for it.
Anyway, I very rarely get physical tickets because I'm a cheapskate and don't want to pay for shipping (same reason why my Dad just emailed them to me). Radio City seems to require physical tickets though so my Moz tickets for that are real tickets and I had to pick up my Florence + the Machine tickets at the door when I saw them there.
Also- the ticket I had is no longer available in case anyone comes by here wanting it.