Don't expect any legitimate "Good Deals". Disney Ticketing will sell at wholesale pricing to several "authorized resellers". The wholesale prices are at about 90% of Gate price. The authorized resellers are allowed to set their own pricing. It would be extremely rare to get as much as 10% off Gate Price for legal tickets.
If you buy from eBay or Craigslist or some of the kiosks on US192 or International Drive there is a strong probability that the tickets are partially used and may not work at all. Note that you cannot tell, by looking at a Disney ticket, what is actually on it, no matter what is printed on it. The only way you can actually tell is by a computer terminal at WDW that is linked to the Automated Ticketing System and can actually read the magnetic stripe, bar code, or RFID chip. The actual use is stored in the computer, not on the ticket.
If you go to the locked sticky Everything About WDW Tickets in the TPA&S Forum, especially Post #20, you can get a lot more information. An easy way to get to the sticky is by clicking on the link in my signature. The ticket sticky is your friend!