ebay has gotten better with their customer protection, but it was lacking the last time I had a problem. I bought a $5 copy of the Matrix on DVD and it was pitted and wouldn't play. I didn't get anything back. However, I did buy a used copy of Mario Strikers Charged off e-bay with no problem.
Typically with e-bay if you are buying non-electronic items (car parts, books, whatever) you don't typically run into a problem. Electronics are a high-scam item, especially if you get those Arabians who have uncles who are princes but they need to send you a $1000 check cause it is the smallest their bank will allow 
Anyways, typically I buy my electronics from Amazon as they have their A-Z protection policy which not only guarantees what Amazon sells, but also their marketplace users (random people who sell on amazon)
EDIT: I agree with makingmusic though. If you have some strict guidlines like that you are less likely to have a problem. However, it still doesn't guarantee anything, as I have heard of people having their account stolen so they could use the good feedback to scam others.
