I think like anything on ebay, if you check the feedback carefully and try to use paypal, you've got some pretty good protection.
You can also use ebay to figure out what else the person has sold in the last few months, but clicking on the links in feedback. If someone has 100 good feedback, and has sold disney tickets before with good comments, then you're probably ok.
If you meet the person at disney to get the tickets, and they were (say) 5 day hoppers with alleged 2 days left, then it's probably a pretty good bet they are still good, particularly if the person is from out of state. What would they still be doing at disney if they didn't have 10 days or so left on the passes?
My problem with doing this is that the cost savings never seems to justify the logistical problems and the risk, no matter how slight.