If the tickets have not been used, they can be re-sold.
Most of the tickets I have seen on eBay, have not been used.
I would not tell someone to Not buy from eBay. I would recommend watching the market for a while, and buying from someone with a lot of feedback (and no negatives for non-delivery or mis-representation of item).
What I don't understand, is the high prices they go for.
Most people have a different risk to savings ratio, which they are willing to accept, than I have.
I admit that I didn't watch the ticket market for more then a few weeks, but I think I saw the general trend.
If you do watch the market, and decide to buy, don't wait until too close to your trip.
I watched the Pal Mickey market for almost 3 months, and then ended up paying above the average selling price anyway (it was getting too close to my trip and I really wanted version 1.1).