Tough battle here... but my vote goes to Amazon, because something about them has always seemed more professional to me than eBay. That whole bidding thing... it's not for me! =P
I vote amazon! Mostly because I've gotten a few hundred dollars of Amazon gift cards from apps and stuff which funded my PC upgrade and other stuffs. And because my mother sells stuff on ebay and because of the changes they've made she went from selling $150+ on an average day to $10.
You used to be able to buy anything on ebay, but now Amazon has caught up and has become even better. I think therefore I shall vote for Amazon, because they always have more than one of everything in case I want to buy two things.