I agree with everyone else. You're taking a BIG gamble. Even if you're dealing with someone that has a high positive feedback rating, what if the person legitimately forgot how many days were left on the passes? Or, what if they were given by a relative/family member to sell, and that person forgot how many days were left?
There is absolutely no way for either the seller or yourself to verify how many days are left until you get to the gates. And, IMO, the last thing you need when you're trying to save a couple bucks is to find out that your pass is a day or two short of what you need or that it won't even work at all.
I've seen Disney World Cast Members post in threads like this before reporting that they have seen entire families in tears at the gate because their passes were invalid.
If you do decide to go the eBay route, just be prepared for the worst. And the worst case scenario is that you may have to shell out hundreds of dollars at the gate in order to get in. Just be wary of any deal that sounds too good to be true.