I know it used to be done, although risky (IMO) to buy partially used park tickets. But now the first time the ticket is used, you have to give a "finger scan", so it is only good for the person who originally scans it. It can't be used by anyone else. Really old tickets (before the finger scanning started) haven't been "assigned" a finger scan yet, so they could be used, if valid. I guess they could sell new, unactivated tickets on ebay, but to me it would be way to risky. Imagine not only getting ripped off, but finding out at the turnstiles that you have been ripped off. What a damper that would be on a great vacation...JMO....
If you subscribe to the Mousesavers newsletter that comes out on the 15th of every month, there is usually a discount for two vendors that are trusted by the website. You can only get the discount through the links on that newsletter. If you sign up now, you can get an idea in a few days of what the discounted tickets would be. I found the best deal from them, comparing the Disney website, and the one linked on the disboards...