When we went on our trip to WDW in December I purposefully purchased more days than we needed so that I could sell them. At that time we were members of the Magic Kingdom Club and got our discounts.
During December E-bay was making some changes in the way they were handling the sales of WDW tickets so I was only able to list them for about 3-4 days. I was able to sell my tickets for a reasonable price and they people who bought them got what they desired (a 2 day Park Hopper ticket).
It helped defray our costs. However, I really don't think it's that much of a savings. You can purchase discount tickets at several online agencies or by joining the Disney Club.
If I had it to do over again, I think I would purchase the most days on a Park Hopper that I could, use the days I needed and save the other days for a future trip (tickets never expire!) Or you could find a friend here on the DIS who is planning a future trip and maybe they would go in with you on the cost of the original ticket, or maybe a friend or relative who may be planning a trip.
Anyway, the point of my posting is that, yes, good reliable sellers do sell their unused portion of their WDW tickets on Ebay. I would definitely check their feedback, email them about specifics and get a feel for the person, and really think about it. If it's what you truly want....
My 2 cents worth...