Buying anything on eBay carries some amount of risk. There are ticket sellers on eBay with lots of positive feedback and many satisfied customers. However, I don't think that their prices are any better than the prices that are charged by the well-known ticketbrokers that are recommended by wdwinfo.com, allearsnet.com and mousesavers.com. There was actually a thread last year started by a DISer who had purchased tickets on eBay from a well-know eBay ticket seller with very good feedback. The DISer did not get her tickets, the seller disappeared from eBay and the buyer had to spend more money to get tickets for her trip. I don't know if she ever got a refund of her eBay money.
Since there is no way to tell how many days are on a ticket until you get to WDW, buying from lesser-known sources is very risky. There are reports of people getting to the gates only to learn that the tickets they hold are not valid. What a lousy way to start a vacation!
The sale of partially-used ticket media is illegal on eBay. And now that Disney's tickets are linked via biometric scan to the original user, buying partially-used tickets is just no a good thing to try.
I highly recommend checking out other places to purchase your tickets and then comparing their prices.