Forget about whether old tickets are still good. All old tickets with days left on them, and all old tickets that have never been used, are good forever.
The issue here is whether you should buy tickets from somebody on eBay when it it impossible to check whether the tickets are any good until the moment you arrive at a Disney park. You can't check them over the phone, you can't check them at the
Disney store, you can't even check them at a Disney resort, you can only check them at Guest Relations or at a ticket window at the parks.
I for one would NEVER buy WDW tickets of any kind from eBay. I would really hate to get to WDW and discover that I had bought a worthless ticket. Not only would my $200 or so be wasted, but I would then have an additional expense on my trip budget to buy real tickets - at the gate price.
Laugh O. Grams, Disney Stores all over still have the old-style park hopper passes left, and will probably keep selling them until they are gone. Ticket brokers like Ticket Mania and Maple Leaf will also have them, and so will
AAA, lots of travel agents, and most of the off-site hotels in the Orlando area. It will take time for all of the old tickets to get sold off.