I would be wary of any tickets on ebay. I buy things from ebay so I'm not saying it's a bad site, but I don't think saving a few dollars is worth the risk in this case. I also don't know that I"d trust the rating system in this case--as soon as someone gets their tickets, they'll leave a good rating for the ebay seller--it got here quickly, great price. But do you know if it worked?? You won't know for sure it's a valid ticket until you are at the gate at WDW trying to use it. What if it's not good?
I have a ticket from last year with a day still on it. I don't even plan on using it for our trip this fall---what if something happened to the magnetic strip and it isn't valid? To me, it's not worth the difference between a 7 day ticket and an 8 day ticket to worry about it.
But, do a search in this forum, you'll find some experiences--there was a thread a while back. Others will chime in, I'm sure!